diff --git a/Lib/distutils/tests/test_build_py.py b/Lib/distutils/tests/test_build_py.py
index 472591d..3c8bc41 100644
--- a/Lib/distutils/tests/test_build_py.py
+++ b/Lib/distutils/tests/test_build_py.py
@@ -16,7 +16,7 @@
                       support.LoggingSilencer,
                       unittest.TestCase):
 
-    def test_package_data(self):
+    def _setup_package_data(self):
         sources = self.mkdtemp()
         f = open(os.path.join(sources, "__init__.py"), "w")
         f.write("# Pretend this is a package.")
@@ -52,10 +52,19 @@
         self.assertEqual(len(cmd.get_outputs()), 3)
         pkgdest = os.path.join(destination, "pkg")
         files = os.listdir(pkgdest)
+        return files
+
+    def test_package_data(self):
+        files = self._setup_package_data()
         self.assertTrue("__init__.py" in files)
-        self.assertTrue("__init__.pyc" in files)
         self.assertTrue("README.txt" in files)
 
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "pyc files are not written with -O2 and above")
+    def test_package_data_pyc(self):
+        files = self._setup_package_data()
+        self.assertTrue("__init__.pyc" in files)
+
     def test_empty_package_dir (self):
         # See SF 1668596/1720897.
         cwd = os.getcwd()
diff --git a/Lib/distutils/tests/test_extension.py b/Lib/distutils/tests/test_extension.py
index cffa9a0..0b5dfb8 100755
--- a/Lib/distutils/tests/test_extension.py
+++ b/Lib/distutils/tests/test_extension.py
@@ -1,6 +1,7 @@
 """Tests for distutils.extension."""
-import unittest
 import os
+import sys
+import unittest
 import warnings
 
 from test.test_support import check_warnings
@@ -32,16 +33,22 @@
 
         self.assertEquals(names, wanted)
 
-    def test_extension_init(self):
-        # the first argument, which is the name, must be a string
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Assertions are omitted with -O2 and above")
+    def test_extension_init_assertions(self):
+        # The first argument, which is the name, must be a string.
         self.assertRaises(AssertionError, Extension, 1, [])
-        ext = Extension('name', [])
-        self.assertEquals(ext.name, 'name')
 
         # the second argument, which is the list of files, must
         # be a list of strings
         self.assertRaises(AssertionError, Extension, 'name', 'file')
         self.assertRaises(AssertionError, Extension, 'name', ['file', 1])
+
+    def test_extension_init(self):
+        ext = Extension('name', [])
+        self.assertEquals(ext.name, 'name')
+
+
         ext = Extension('name', ['file1', 'file2'])
         self.assertEquals(ext.sources, ['file1', 'file2'])
 
diff --git a/Lib/distutils/tests/test_install_lib.py b/Lib/distutils/tests/test_install_lib.py
index 99a6d90..13d27ab 100644
--- a/Lib/distutils/tests/test_install_lib.py
+++ b/Lib/distutils/tests/test_install_lib.py
@@ -1,6 +1,6 @@
 """Tests for distutils.command.install_data."""
-import sys
 import os
+import sys
 import unittest
 
 from distutils.command.install_lib import install_lib
@@ -31,9 +31,7 @@
         cmd.finalize_options()
         self.assertEquals(cmd.optimize, 2)
 
-    @unittest.skipUnless(not sys.dont_write_bytecode,
-                         'byte-compile not supported')
-    def test_byte_compile(self):
+    def _setup_byte_compile(self):
         pkg_dir, dist = self.create_dist()
         cmd = install_lib(dist)
         cmd.compile = cmd.optimize = 1
@@ -41,8 +39,15 @@
         f = os.path.join(pkg_dir, 'foo.py')
         self.write_file(f, '# python file')
         cmd.byte_compile([f])
-        self.assertTrue(os.path.exists(os.path.join(pkg_dir, 'foo.pyc')))
-        self.assertTrue(os.path.exists(os.path.join(pkg_dir, 'foo.pyo')))
+        return pkg_dir
+
+    @unittest.skipIf(sys.dont_write_bytecode, 'byte-compile not enabled')
+    def test_byte_compile(self):
+        pkg_dir = self._setup_byte_compile()
+        if sys.flags.optimize < 1:
+            self.assertTrue(os.path.exists(os.path.join(pkg_dir, 'foo.pyc')))
+        else:
+            self.assertTrue(os.path.exists(os.path.join(pkg_dir, 'foo.pyo')))
 
     def test_get_outputs(self):
         pkg_dir, dist = self.create_dist()
diff --git a/Lib/doctest.py b/Lib/doctest.py
index b07ef6e..41eccbe 100644
--- a/Lib/doctest.py
+++ b/Lib/doctest.py
@@ -2240,6 +2240,19 @@
     def shortDescription(self):
         return "Doctest: " + self._dt_test.name
 
+class SkipDocTestCase(DocTestCase):
+    def __init__(self):
+        DocTestCase.__init__(self, None)
+
+    def setUp(self):
+        self.skipTest("DocTestSuite will not work with -O2 and above")
+
+    def test_skip(self):
+        pass
+
+    def shortDescription(self):
+        return "Skipping tests from %s" % module.__name__
+
 def DocTestSuite(module=None, globs=None, extraglobs=None, test_finder=None,
                  **options):
     """
@@ -2282,13 +2295,20 @@
 
     module = _normalize_module(module)
     tests = test_finder.find(module, globs=globs, extraglobs=extraglobs)
-    if not tests:
+
+    if not tests and sys.flags.optimize >=2:
+        # Skip doctests when running with -O2
+        suite = unittest.TestSuite()
+        suite.addTest(SkipDocTestCase())
+        return suite
+    elif not tests:
         # Why do we want to do this? Because it reveals a bug that might
         # otherwise be hidden.
         raise ValueError(module, "has no tests")
 
     tests.sort()
     suite = unittest.TestSuite()
+
     for test in tests:
         if len(test.examples) == 0:
             continue
diff --git a/Lib/lib2to3/tests/test_refactor.py b/Lib/lib2to3/tests/test_refactor.py
index 1729027..de4b29a 100644
--- a/Lib/lib2to3/tests/test_refactor.py
+++ b/Lib/lib2to3/tests/test_refactor.py
@@ -239,6 +239,9 @@
         finally:
             os.linesep = old_sep
 
+
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
     def test_refactor_docstring(self):
         rt = self.rt()
 
diff --git a/Lib/test/test_collections.py b/Lib/test/test_collections.py
index 77ce7ed..4246b23 100644
--- a/Lib/test/test_collections.py
+++ b/Lib/test/test_collections.py
@@ -8,6 +8,7 @@
 from random import randrange, shuffle
 import keyword
 import re
+import sys
 from collections import Hashable, Iterable, Iterator
 from collections import Sized, Container, Callable
 from collections import Set, MutableSet
@@ -21,7 +22,6 @@
     def test_factory(self):
         Point = namedtuple('Point', 'x y')
         self.assertEqual(Point.__name__, 'Point')
-        self.assertEqual(Point.__doc__, 'Point(x, y)')
         self.assertEqual(Point.__slots__, ())
         self.assertEqual(Point.__module__, __name__)
         self.assertEqual(Point.__getitem__, tuple.__getitem__)
@@ -48,6 +48,12 @@
         self.assertRaises(TypeError, Point._make, [11])                     # catch too few args
         self.assertRaises(TypeError, Point._make, [11, 22, 33])             # catch too many args
 
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
+    def test_factory_doc_attr(self):
+        Point = namedtuple('Point', 'x y')
+        self.assertEqual(Point.__doc__, 'Point(x, y)')
+
     def test_name_fixer(self):
         for spec, renamed in [
             [('efg', 'g%hi'),  ('efg', '_1')],                              # field with non-alpha char
diff --git a/Lib/test/test_contextlib.py b/Lib/test/test_contextlib.py
index 6350717..4d233da 100644
--- a/Lib/test/test_contextlib.py
+++ b/Lib/test/test_contextlib.py
@@ -2,6 +2,7 @@
 
 
 import os
+import sys
 import tempfile
 import unittest
 import threading
@@ -84,7 +85,7 @@
             raise ZeroDivisionError(999)
         self.assertEqual(state, [1, 42, 999])
 
-    def test_contextmanager_attribs(self):
+    def _create_contextmanager_attribs(self):
         def attribs(**kw):
             def decorate(func):
                 for k,v in kw.items():
@@ -95,8 +96,17 @@
         @attribs(foo='bar')
         def baz(spam):
             """Whee!"""
+        return baz
+
+    def test_contextmanager_attribs(self):
+        baz = self._create_contextmanager_attribs()
         self.assertEqual(baz.__name__,'baz')
         self.assertEqual(baz.foo, 'bar')
+
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
+    def test_contextmanager_doc_attrib(self):
+        baz = self._create_contextmanager_attribs()
         self.assertEqual(baz.__doc__, "Whee!")
 
 class NestedTestCase(unittest.TestCase):
diff --git a/Lib/test/test_descr.py b/Lib/test/test_descr.py
index 8ec03087..536d70a 100644
--- a/Lib/test/test_descr.py
+++ b/Lib/test/test_descr.py
@@ -2057,6 +2057,9 @@
         else:
             self.fail("expected ZeroDivisionError from bad property")
 
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
+    def test_properties_doc_attrib(self):
         class E(object):
             def getter(self):
                 "getter method"
@@ -2069,6 +2072,7 @@
             prop2 = property(fset=setter)
             self.assertEqual(prop2.__doc__, None)
 
+    def test_testcapi_no_segfault(self):
         # this segfaulted in 2.5b2
         try:
             import _testcapi
diff --git a/Lib/test/test_doctest2.py b/Lib/test/test_doctest2.py
index 865a32e..8c043ba 100644
--- a/Lib/test/test_doctest2.py
+++ b/Lib/test/test_doctest2.py
@@ -12,7 +12,11 @@
 
 """
 
+import sys
+import unittest
 from test import test_support
+if sys.flags.optimize >= 2:
+    raise unittest.SkipTest("Cannot test docstrings with -O2")
 
 class C(object):
     u"""Class C.
diff --git a/Lib/test/test_docxmlrpc.py b/Lib/test/test_docxmlrpc.py
index dae4109..1af3433 100644
--- a/Lib/test/test_docxmlrpc.py
+++ b/Lib/test/test_docxmlrpc.py
@@ -1,5 +1,6 @@
 from DocXMLRPCServer import DocXMLRPCServer
 import httplib
+import sys
 from test import test_support
 import threading
 import time
@@ -8,6 +9,20 @@
 
 PORT = None
 
+def make_request_and_skipIf(condition, reason):
+    # If we skip the test, we have to make a request because the
+    # the server created in setUp blocks expecting one to come in.
+    if not condition:
+        return lambda func: func
+    def decorator(func):
+        def make_request_and_skip(self):
+            self.client.request("GET", "/")
+            self.client.getresponse()
+            raise unittest.SkipTest(reason)
+        return make_request_and_skip
+    return decorator
+
+
 def server(evt, numrequests):
     serv = DocXMLRPCServer(("localhost", 0), logRequests=False)
 
@@ -111,10 +126,12 @@
                       '&lt;lambda&gt;</strong></a>(x, y)</dt></dl>',
                       response.read())
 
+    @make_request_and_skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
     def test_autolinking(self):
-        """Test that the server correctly automatically wraps references to PEPS
-        and RFCs with links, and that it linkifies text starting with http or
-        ftp protocol prefixes.
+        """Test that the server correctly automatically wraps references to
+        PEPS and RFCs with links, and that it linkifies text starting with
+        http or ftp protocol prefixes.
 
         The documentation for the "add" method contains the test material.
         """
@@ -133,11 +150,13 @@
              'auto-linked,&nbsp;too:<br>\n<a href="http://google.com">'
              'http://google.com</a>.</tt></dd></dl>'), response.read())
 
+    @make_request_and_skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
     def test_system_methods(self):
         """Test the precense of three consecutive system.* methods.
 
-        This also tests their use of parameter type recognition and the systems
-        related to that process.
+        This also tests their use of parameter type recognition and the
+        systems related to that process.
         """
         self.client.request("GET", "/")
         response = self.client.getresponse()
diff --git a/Lib/test/test_functools.py b/Lib/test/test_functools.py
index b88e9b7..2549e05 100644
--- a/Lib/test/test_functools.py
+++ b/Lib/test/test_functools.py
@@ -1,4 +1,5 @@
 import functools
+import sys
 import unittest
 from test import test_support
 from weakref import proxy
@@ -179,7 +180,7 @@
             for key in wrapped_attr:
                 self.assertTrue(wrapped_attr[key] is wrapper_attr[key])
 
-    def test_default_update(self):
+    def _default_update(self):
         def f():
             """This is a test"""
             pass
@@ -187,11 +188,20 @@
         def wrapper():
             pass
         functools.update_wrapper(wrapper, f)
+        return wrapper, f
+
+    def test_default_update(self):
+        wrapper, f = self._default_update()
         self.check_wrapper(wrapper, f)
         self.assertEqual(wrapper.__name__, 'f')
-        self.assertEqual(wrapper.__doc__, 'This is a test')
         self.assertEqual(wrapper.attr, 'This is also a test')
 
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
+    def test_default_update_doc(self):
+        wrapper, f = self._default_update()
+        self.assertEqual(wrapper.__doc__, 'This is a test')
+
     def test_no_update(self):
         def f():
             """This is a test"""
@@ -232,7 +242,7 @@
 
 class TestWraps(TestUpdateWrapper):
 
-    def test_default_update(self):
+    def _default_update(self):
         def f():
             """This is a test"""
             pass
@@ -241,10 +251,19 @@
         def wrapper():
             pass
         self.check_wrapper(wrapper, f)
+        return wrapper
+
+    def test_default_update(self):
+        wrapper = self._default_update()
         self.assertEqual(wrapper.__name__, 'f')
-        self.assertEqual(wrapper.__doc__, 'This is a test')
         self.assertEqual(wrapper.attr, 'This is also a test')
 
+    @unittest.skipIf(not sys.flags.optimize <= 1,
+                     "Docstrings are omitted with -O2 and above")
+    def test_default_update_doc(self):
+        wrapper = self._default_update()
+        self.assertEqual(wrapper.__doc__, 'This is a test')
+
     def test_no_update(self):
         def f():
             """This is a test"""
@@ -323,7 +342,6 @@
 
 
 def test_main(verbose=None):
-    import sys
     test_classes = (
         TestPartial,
         TestPartialSubclass,
diff --git a/Lib/test/test_inspect.py b/Lib/test/test_inspect.py
index 451d309..e607af2 100644
--- a/Lib/test/test_inspect.py
+++ b/Lib/test/test_inspect.py
@@ -230,6 +230,8 @@
         self.assertEqual(functions, [('eggs', mod.eggs),
                                      ('spam', mod.spam)])
 
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
     def test_getdoc(self):
         self.assertEqual(inspect.getdoc(mod), 'A module docstring.')
         self.assertEqual(inspect.getdoc(mod.StupidGit),
diff --git a/Lib/test/test_pkg.py b/Lib/test/test_pkg.py
index e3b86e6..6fb3f3d 100644
--- a/Lib/test/test_pkg.py
+++ b/Lib/test/test_pkg.py
@@ -51,7 +51,8 @@
 
     def tearDown(self):
         sys.path[:] = self.syspath
-        cleanout(self.root)
+        if self.root: # Only clean if the test was actually run
+            cleanout(self.root)
 
         # delete all modules concerning the tested hiearchy
         if self.pkgname:
@@ -101,9 +102,6 @@
         ]
         self.mkhier(hier)
 
-        import t2
-        self.assertEqual(t2.__doc__, "doc for t2")
-
         import t2.sub
         import t2.sub.subsub
         self.assertEqual(t2.__name__, "t2")
@@ -274,6 +272,17 @@
         self.assertFalse(sub)
         self.assertFalse(subsub)
 
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
+    def test_8(self):
+        hier = [
+                ("t8", None),
+                ("t8 __init__"+os.extsep+"py", "'doc for t8'"),
+               ]
+        self.mkhier(hier)
+
+        import t8
+        self.assertEqual(t8.__doc__, "doc for t8")
 
 def test_main():
     test_support.run_unittest(__name__)
diff --git a/Lib/test/test_property.py b/Lib/test/test_property.py
index 684f8fe..6d6bece 100644
--- a/Lib/test/test_property.py
+++ b/Lib/test/test_property.py
@@ -1,6 +1,7 @@
 # Test case for property
 # more tests are in test_descr
 
+import sys
 import unittest
 from test.test_support import run_unittest
 
@@ -91,7 +92,6 @@
         base.spam = 20
         self.assertEqual(base.spam, 20)
         self.assertEqual(base._spam, 20)
-        self.assertEqual(base.__class__.spam.__doc__, "BaseClass.getter")
 
     def test_property_decorator_subclass(self):
         # see #1620
@@ -99,14 +99,27 @@
         self.assertRaises(PropertyGet, getattr, sub, "spam")
         self.assertRaises(PropertySet, setattr, sub, "spam", None)
         self.assertRaises(PropertyDel, delattr, sub, "spam")
+
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
+    def test_property_decorator_subclass_doc(self):
+        sub = SubClass()
         self.assertEqual(sub.__class__.spam.__doc__, "SubClass.getter")
 
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
+    def test_property_decorator_baseclass_doc(self):
+        base = BaseClass()
+        self.assertEqual(base.__class__.spam.__doc__, "BaseClass.getter")
+
     def test_property_decorator_doc(self):
         base = PropertyDocBase()
         sub = PropertyDocSub()
         self.assertEqual(base.__class__.spam.__doc__, "spam spam spam")
         self.assertEqual(sub.__class__.spam.__doc__, "spam spam spam")
 
+    @unittest.skipIf(sys.flags.optimize >= 1,
+                     "Docstrings are omitted with -O2 and above")
     def test_property_getter_doc_override(self):
         newgettersub = PropertySubNewGetter()
         self.assertEqual(newgettersub.spam, 5)
@@ -126,16 +139,6 @@
 
 class PropertySubclassTests(unittest.TestCase):
 
-    def test_docstring_copy(self):
-        class Foo(object):
-            @PropertySub
-            def spam(self):
-                """spam wrapped in property subclass"""
-                return 1
-        self.assertEqual(
-            Foo.spam.__doc__,
-            "spam wrapped in property subclass")
-
     def test_slots_docstring_copy_exception(self):
         try:
             class Foo(object):
@@ -148,6 +151,20 @@
         else:
             raise Exception("AttributeError not raised")
 
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
+    def test_docstring_copy(self):
+        class Foo(object):
+            @PropertySub
+            def spam(self):
+                """spam wrapped in property subclass"""
+                return 1
+        self.assertEqual(
+            Foo.spam.__doc__,
+            "spam wrapped in property subclass")
+
+    @unittest.skipIf(sys.flags.optimize <= 2,
+                     "Docstrings are omitted with -O2 and above")
     def test_property_setter_copies_getter_docstring(self):
         class Foo(object):
             def __init__(self): self._spam = 1
@@ -179,6 +196,8 @@
             FooSub.spam.__doc__,
             "spam wrapped in property subclass")
 
+    @unittest.skipIf(sys.flags.optimize <= 2,
+                     "Docstrings are omitted with -O2 and above")
     def test_property_new_getter_new_docstring(self):
 
         class Foo(object):
diff --git a/Lib/test/test_pydoc.py b/Lib/test/test_pydoc.py
index 5f2d3b3..43ad2c1 100644
--- a/Lib/test/test_pydoc.py
+++ b/Lib/test/test_pydoc.py
@@ -219,6 +219,8 @@
 
 class PyDocDocTest(unittest.TestCase):
 
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
     def test_html_doc(self):
         result, doc_loc = get_pydoc_html(pydoc_mod)
         mod_file = inspect.getabsfile(pydoc_mod)
@@ -232,6 +234,8 @@
             print_diffs(expected_html, result)
             self.fail("outputs are not equal, see diff above")
 
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
     def test_text_doc(self):
         result, doc_loc = get_pydoc_text(pydoc_mod)
         expected_text = expected_text_pattern % \
diff --git a/Lib/test/test_unittest.py b/Lib/test/test_unittest.py
index d5f9a5c..d009bf9 100644
--- a/Lib/test/test_unittest.py
+++ b/Lib/test/test_unittest.py
@@ -2041,6 +2041,8 @@
                 'testGetDescriptionWithoutDocstring (' + __name__ +
                 '.Test_TestResult)')
 
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
     def testGetDescriptionWithOneLineDocstring(self):
         """Tests getDescription() for a method with a docstring."""
         result = unittest.TextTestResult(None, True, 1)
@@ -2050,6 +2052,8 @@
                 '(' + __name__ + '.Test_TestResult)\n'
                 'Tests getDescription() for a method with a docstring.'))
 
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
     def testGetDescriptionWithMultiLineDocstring(self):
         """Tests getDescription() for a method with a longer docstring.
         The second line of the docstring.
@@ -2525,12 +2529,16 @@
     def testShortDescriptionWithoutDocstring(self):
         self.assertIsNone(self.shortDescription())
 
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
     def testShortDescriptionWithOneLineDocstring(self):
         """Tests shortDescription() for a method with a docstring."""
         self.assertEqual(
                 self.shortDescription(),
                 'Tests shortDescription() for a method with a docstring.')
 
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
     def testShortDescriptionWithMultiLineDocstring(self):
         """Tests shortDescription() for a method with a longer docstring.
 
diff --git a/Lib/test/test_xmlrpc.py b/Lib/test/test_xmlrpc.py
index 9a8cc55..94c5825 100644
--- a/Lib/test/test_xmlrpc.py
+++ b/Lib/test/test_xmlrpc.py
@@ -505,6 +505,8 @@
                 # protocol error; provide additional information in test output
                 self.fail("%s\n%s" % (e, getattr(e, "headers", "")))
 
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
     def test_introspection3(self):
         try:
             # test native doc
diff --git a/Misc/ACKS b/Misc/ACKS
index 3fb65e0..426d0dd 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -755,6 +755,7 @@
 Frank J. Tobin
 R Lindsay Todd
 Bennett Todd
+Matias Torchinsky
 Richard Townsend
 Laurence Tratt
 John Tromp
diff --git a/Misc/NEWS b/Misc/NEWS
index 872affe..c3cf9b5 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -57,6 +57,9 @@
 Tests
 -----
 
+- Issue #6292: for the moment at least, the test suite runs cleanly if python
+  is run with the -OO flag.  Tests requiring docstrings are skipped.
+
 - Issue #7712: test_support gained a new `temp_cwd` context manager which is
   now also used by regrtest to run all the tests in a temporary directory.
   The original CWD is saved in `test_support.SAVEDCWD`.