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.

tests/test_docstring_options.py

@@ -1,42 +1,38 @@
+from pybind11_tests import docstring_options as m
 
 
 def test_docstring_options():
-    from pybind11_tests import (test_function1, test_function2, test_function3,
-                                test_function4, test_function5, test_function6,
-                                test_function7, DocstringTestFoo,
-                                test_overloaded1, test_overloaded2, test_overloaded3)
-
     # options.disable_function_signatures()
-    assert not test_function1.__doc__
+    assert not m.test_function1.__doc__
 
-    assert test_function2.__doc__ == "A custom docstring"
+    assert m.test_function2.__doc__ == "A custom docstring"
 
     # docstring specified on just the first overload definition:
-    assert test_overloaded1.__doc__ == "Overload docstring"
+    assert m.test_overloaded1.__doc__ == "Overload docstring"
 
     # docstring on both overloads:
-    assert test_overloaded2.__doc__ == "overload docstring 1\noverload docstring 2"
+    assert m.test_overloaded2.__doc__ == "overload docstring 1\noverload docstring 2"
 
     # docstring on only second overload:
-    assert test_overloaded3.__doc__ == "Overload docstr"
+    assert m.test_overloaded3.__doc__ == "Overload docstr"
 
     # options.enable_function_signatures()
-    assert test_function3.__doc__ .startswith("test_function3(a: int, b: int) -> None")
+    assert m.test_function3.__doc__ .startswith("test_function3(a: int, b: int) -> None")
 
-    assert test_function4.__doc__ .startswith("test_function4(a: int, b: int) -> None")
-    assert test_function4.__doc__ .endswith("A custom docstring\n")
+    assert m.test_function4.__doc__ .startswith("test_function4(a: int, b: int) -> None")
+    assert m.test_function4.__doc__ .endswith("A custom docstring\n")
 
     # options.disable_function_signatures()
     # options.disable_user_defined_docstrings()
-    assert not test_function5.__doc__
+    assert not m.test_function5.__doc__
 
     # nested options.enable_user_defined_docstrings()
-    assert test_function6.__doc__ == "A custom docstring"
+    assert m.test_function6.__doc__ == "A custom docstring"
 
     # RAII destructor
-    assert test_function7.__doc__ .startswith("test_function7(a: int, b: int) -> None")
-    assert test_function7.__doc__ .endswith("A custom docstring\n")
+    assert m.test_function7.__doc__ .startswith("test_function7(a: int, b: int) -> None")
+    assert m.test_function7.__doc__ .endswith("A custom docstring\n")
 
     # Suppression of user-defined docstrings for non-function objects
-    assert not DocstringTestFoo.__doc__
-    assert not DocstringTestFoo.value_prop.__doc__
+    assert not m.DocstringTestFoo.__doc__
+    assert not m.DocstringTestFoo.value_prop.__doc__