Issue #10992: make tests pass when run under coverage.

Various tests fail when run under coverage. A primary culprit is refcount tests
which fail as the counts are thrown off by the coverage code. A new decorator
-- test.support.refcount_test -- is used to decorate tests which test refcounts
and to skip them when running under coverage. Other tests simply fail because
of changes in the system (e.g., __local__ suddenly appearing).

Thanks to Kristian Vlaardingerbroek for helping to diagnose the test failures.
diff --git a/Lib/test/test_metaclass.py b/Lib/test/test_metaclass.py
index 219ab99..6862900 100644
--- a/Lib/test/test_metaclass.py
+++ b/Lib/test/test_metaclass.py
@@ -246,7 +246,13 @@
 
 """
 
-__test__ = {'doctests' : doctests}
+import sys
+
+# Trace function introduces __locals__ which causes various tests to fail.
+if hasattr(sys, 'gettrace') and sys.gettrace():
+    __test__ = {}
+else:
+    __test__ = {'doctests' : doctests}
 
 def test_main(verbose=False):
     from test import support