Update all remaining tests to new test styles

This udpates all the remaining tests to the new test suite code and
comment styles started in #898.  For the most part, the test coverage
here is unchanged, with a few minor exceptions as noted below.

- test_constants_and_functions: this adds more overload tests with
  overloads with different number of arguments for more comprehensive
  overload_cast testing.  The test style conversion broke the overload
  tests under MSVC 2015, prompting the additional tests while looking
  for a workaround.

- test_eigen: this dropped the unused functions `get_cm_corners` and
  `get_cm_corners_const`--these same tests were duplicates of the same
  things provided (and used) via ReturnTester methods.

- test_opaque_types: this test had a hidden dependence on ExampleMandA
  which is now fixed by using the global UserType which suffices for the
  relevant test.

- test_methods_and_attributes: this required some additions to UserType
  to make it usable as a replacement for the test's previous SimpleType:
  UserType gained a value mutator, and the `value` property is not
  mutable (it was previously readonly).  Some overload tests were also
  added to better test overload_cast (as described above).

- test_numpy_array: removed the untemplated mutate_data/mutate_data_t:
  the templated versions with an empty parameter pack expand to the same
  thing.

- test_stl: this was already mostly in the new style; this just tweaks
  things a bit, localizing a class, and adding some missing
  `// test_whatever` comments.

- test_virtual_functions: like `test_stl`, this was mostly in the new
  test style already, but needed some `// test_whatever` comments.
  This commit also moves the inherited virtual example code to the end
  of the file, after the main set of tests (since it is less important
  than the other tests, and rather length); it also got renamed to
  `test_inherited_virtuals` (from `test_inheriting_repeat`) because it
  tests both inherited virtual approaches, not just the repeat approach.
diff --git a/tests/test_call_policies.py b/tests/test_call_policies.py
index 42d0ffd..6c60f81 100644
--- a/tests/test_call_policies.py
+++ b/tests/test_call_policies.py
@@ -1,15 +1,15 @@
 import pytest
+from pybind11_tests import call_policies as m
+from pybind11_tests import ConstructorStats
 
 
 def test_keep_alive_argument(capture):
-    from pybind11_tests import Parent, Child, ConstructorStats
-
     n_inst = ConstructorStats.detail_reg_inst()
     with capture:
-        p = Parent()
+        p = m.Parent()
     assert capture == "Allocating parent."
     with capture:
-        p.addChild(Child())
+        p.addChild(m.Child())
         assert ConstructorStats.detail_reg_inst() == n_inst + 1
     assert capture == """
         Allocating child.
@@ -21,10 +21,10 @@
     assert capture == "Releasing parent."
 
     with capture:
-        p = Parent()
+        p = m.Parent()
     assert capture == "Allocating parent."
     with capture:
-        p.addChildKeepAlive(Child())
+        p.addChildKeepAlive(m.Child())
         assert ConstructorStats.detail_reg_inst() == n_inst + 2
     assert capture == "Allocating child."
     with capture:
@@ -37,11 +37,9 @@
 
 
 def test_keep_alive_return_value(capture):
-    from pybind11_tests import Parent, ConstructorStats
-
     n_inst = ConstructorStats.detail_reg_inst()
     with capture:
-        p = Parent()
+        p = m.Parent()
     assert capture == "Allocating parent."
     with capture:
         p.returnChild()
@@ -56,7 +54,7 @@
     assert capture == "Releasing parent."
 
     with capture:
-        p = Parent()
+        p = m.Parent()
     assert capture == "Allocating parent."
     with capture:
         p.returnChildKeepAlive()
@@ -74,11 +72,9 @@
 # https://bitbucket.org/pypy/pypy/issues/2447
 @pytest.unsupported_on_pypy
 def test_alive_gc(capture):
-    from pybind11_tests import ParentGC, Child, ConstructorStats
-
     n_inst = ConstructorStats.detail_reg_inst()
-    p = ParentGC()
-    p.addChildKeepAlive(Child())
+    p = m.ParentGC()
+    p.addChildKeepAlive(m.Child())
     assert ConstructorStats.detail_reg_inst() == n_inst + 2
     lst = [p]
     lst.append(lst)   # creates a circular reference
@@ -92,14 +88,12 @@
 
 
 def test_alive_gc_derived(capture):
-    from pybind11_tests import Parent, Child, ConstructorStats
-
-    class Derived(Parent):
+    class Derived(m.Parent):
         pass
 
     n_inst = ConstructorStats.detail_reg_inst()
     p = Derived()
-    p.addChildKeepAlive(Child())
+    p.addChildKeepAlive(m.Child())
     assert ConstructorStats.detail_reg_inst() == n_inst + 2
     lst = [p]
     lst.append(lst)   # creates a circular reference
@@ -113,16 +107,14 @@
 
 
 def test_alive_gc_multi_derived(capture):
-    from pybind11_tests import Parent, Child, ConstructorStats
-
-    class Derived(Parent, Child):
+    class Derived(m.Parent, m.Child):
         def __init__(self):
-            Parent.__init__(self)
-            Child.__init__(self)
+            m.Parent.__init__(self)
+            m.Child.__init__(self)
 
     n_inst = ConstructorStats.detail_reg_inst()
     p = Derived()
-    p.addChildKeepAlive(Child())
+    p.addChildKeepAlive(m.Child())
     # +3 rather than +2 because Derived corresponds to two registered instances
     assert ConstructorStats.detail_reg_inst() == n_inst + 3
     lst = [p]
@@ -138,11 +130,9 @@
 
 
 def test_return_none(capture):
-    from pybind11_tests import Parent, ConstructorStats
-
     n_inst = ConstructorStats.detail_reg_inst()
     with capture:
-        p = Parent()
+        p = m.Parent()
     assert capture == "Allocating parent."
     with capture:
         p.returnNullChildKeepAliveChild()
@@ -154,7 +144,7 @@
     assert capture == "Releasing parent."
 
     with capture:
-        p = Parent()
+        p = m.Parent()
     assert capture == "Allocating parent."
     with capture:
         p.returnNullChildKeepAliveParent()
@@ -167,14 +157,12 @@
 
 
 def test_call_guard():
-    from pybind11_tests import call_policies
+    assert m.unguarded_call() == "unguarded"
+    assert m.guarded_call() == "guarded"
 
-    assert call_policies.unguarded_call() == "unguarded"
-    assert call_policies.guarded_call() == "guarded"
+    assert m.multiple_guards_correct_order() == "guarded & guarded"
+    assert m.multiple_guards_wrong_order() == "unguarded & guarded"
 
-    assert call_policies.multiple_guards_correct_order() == "guarded & guarded"
-    assert call_policies.multiple_guards_wrong_order() == "unguarded & guarded"
-
-    if hasattr(call_policies, "with_gil"):
-        assert call_policies.with_gil() == "GIL held"
-        assert call_policies.without_gil() == "GIL released"
+    if hasattr(m, "with_gil"):
+        assert m.with_gil() == "GIL held"
+        assert m.without_gil() == "GIL released"