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_buffers.py

@@ -1,6 +1,7 @@
 import struct
 import pytest
-from pybind11_tests import Matrix, ConstructorStats, PTMFBuffer, ConstPTMFBuffer, DerivedPTMFBuffer
+from pybind11_tests import buffers as m
+from pybind11_tests import ConstructorStats
 
 pytestmark = pytest.requires_numpy
 
@@ -10,17 +11,17 @@ with pytest.suppress(ImportError):
 
 def test_from_python():
     with pytest.raises(RuntimeError) as excinfo:
-        Matrix(np.array([1, 2, 3]))  # trying to assign a 1D array
+        m.Matrix(np.array([1, 2, 3]))  # trying to assign a 1D array
     assert str(excinfo.value) == "Incompatible buffer format!"
 
     m3 = np.array([[1, 2, 3], [4, 5, 6]]).astype(np.float32)
-    m4 = Matrix(m3)
+    m4 = m.Matrix(m3)
 
     for i in range(m4.rows()):
         for j in range(m4.cols()):
             assert m3[i, j] == m4[i, j]
 
-    cstats = ConstructorStats.get(Matrix)
+    cstats = ConstructorStats.get(m.Matrix)
     assert cstats.alive() == 1
     del m3, m4
     assert cstats.alive() == 0
@@ -35,26 +36,26 @@ def test_from_python():
 # https://bitbucket.org/pypy/pypy/issues/2444
 @pytest.unsupported_on_pypy
 def test_to_python():
-    m = Matrix(5, 5)
-    assert memoryview(m).shape == (5, 5)
+    mat = m.Matrix(5, 5)
+    assert memoryview(mat).shape == (5, 5)
 
-    assert m[2, 3] == 0
-    m[2, 3] = 4
-    assert m[2, 3] == 4
+    assert mat[2, 3] == 0
+    mat[2, 3] = 4
+    assert mat[2, 3] == 4
 
-    m2 = np.array(m, copy=False)
-    assert m2.shape == (5, 5)
-    assert abs(m2).sum() == 4
-    assert m2[2, 3] == 4
-    m2[2, 3] = 5
-    assert m2[2, 3] == 5
+    mat2 = np.array(mat, copy=False)
+    assert mat2.shape == (5, 5)
+    assert abs(mat2).sum() == 4
+    assert mat2[2, 3] == 4
+    mat2[2, 3] = 5
+    assert mat2[2, 3] == 5
 
-    cstats = ConstructorStats.get(Matrix)
+    cstats = ConstructorStats.get(m.Matrix)
     assert cstats.alive() == 1
-    del m
+    del mat
     pytest.gc_collect()
     assert cstats.alive() == 1
-    del m2  # holds an m reference
+    del mat2  # holds a mat reference
     pytest.gc_collect()
     assert cstats.alive() == 0
     assert cstats.values() == ["5x5 matrix"]
@@ -67,16 +68,15 @@ def test_to_python():
 @pytest.unsupported_on_pypy
 def test_inherited_protocol():
     """SquareMatrix is derived from Matrix and inherits the buffer protocol"""
-    from pybind11_tests import SquareMatrix
 
-    matrix = SquareMatrix(5)
+    matrix = m.SquareMatrix(5)
     assert memoryview(matrix).shape == (5, 5)
     assert np.asarray(matrix).shape == (5, 5)
 
 
 @pytest.unsupported_on_pypy
-def test_ptmf():
-    for cls in [PTMFBuffer, ConstPTMFBuffer, DerivedPTMFBuffer]:
+def test_pointer_to_member_fn():
+    for cls in [m.Buffer, m.ConstBuffer, m.DerivedBuffer]:
         buf = cls()
         buf.value = 0x12345678
         value = struct.unpack('i', bytearray(buf))[0]