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_numpy_vectorize.py

@@ -1,4 +1,5 @@
 import pytest
+from pybind11_tests import numpy_vectorize as m
 
 pytestmark = pytest.requires_numpy
 
@@ -7,11 +8,9 @@ with pytest.suppress(ImportError):
 
 
 def test_vectorize(capture):
-    from pybind11_tests import vectorized_func, vectorized_func2, vectorized_func3
+    assert np.isclose(m.vectorized_func3(np.array(3 + 7j)), [6 + 14j])
 
-    assert np.isclose(vectorized_func3(np.array(3 + 7j)), [6 + 14j])
-
-    for f in [vectorized_func, vectorized_func2]:
+    for f in [m.vectorized_func, m.vectorized_func2]:
         with capture:
             assert np.isclose(f(1, 2, 3), 6)
         assert capture == "my_func(x:int=1, y:float=2, z:float=3)"
@@ -103,23 +102,19 @@ def test_vectorize(capture):
 
 
 def test_type_selection():
-    from pybind11_tests import selective_func
-
-    assert selective_func(np.array([1], dtype=np.int32)) == "Int branch taken."
-    assert selective_func(np.array([1.0], dtype=np.float32)) == "Float branch taken."
-    assert selective_func(np.array([1.0j], dtype=np.complex64)) == "Complex float branch taken."
+    assert m.selective_func(np.array([1], dtype=np.int32)) == "Int branch taken."
+    assert m.selective_func(np.array([1.0], dtype=np.float32)) == "Float branch taken."
+    assert m.selective_func(np.array([1.0j], dtype=np.complex64)) == "Complex float branch taken."
 
 
 def test_docs(doc):
-    from pybind11_tests import vectorized_func
-
-    assert doc(vectorized_func) == """
+    assert doc(m.vectorized_func) == """
         vectorized_func(arg0: numpy.ndarray[int32], arg1: numpy.ndarray[float32], arg2: numpy.ndarray[float64]) -> object
     """  # noqa: E501 line too long
 
 
 def test_trivial_broadcasting():
-    from pybind11_tests import vectorized_is_trivial, trivial, vectorized_func
+    trivial, vectorized_is_trivial = m.trivial, m.vectorized_is_trivial
 
     assert vectorized_is_trivial(1, 2, 3) == trivial.c_trivial
     assert vectorized_is_trivial(np.array(1), np.array(2), 3) == trivial.c_trivial
@@ -153,51 +148,49 @@ def test_trivial_broadcasting():
     assert vectorized_is_trivial(z1[1::4, 1::4], y2, 1) == trivial.f_trivial
     assert vectorized_is_trivial(y1[1::4, 1::4], z2, 1) == trivial.c_trivial
 
-    assert vectorized_func(z1, z2, z3).flags.c_contiguous
-    assert vectorized_func(y1, y2, y3).flags.f_contiguous
-    assert vectorized_func(z1, 1, 1).flags.c_contiguous
-    assert vectorized_func(1, y2, 1).flags.f_contiguous
-    assert vectorized_func(z1[1::4, 1::4], y2, 1).flags.f_contiguous
-    assert vectorized_func(y1[1::4, 1::4], z2, 1).flags.c_contiguous
+    assert m.vectorized_func(z1, z2, z3).flags.c_contiguous
+    assert m.vectorized_func(y1, y2, y3).flags.f_contiguous
+    assert m.vectorized_func(z1, 1, 1).flags.c_contiguous
+    assert m.vectorized_func(1, y2, 1).flags.f_contiguous
+    assert m.vectorized_func(z1[1::4, 1::4], y2, 1).flags.f_contiguous
+    assert m.vectorized_func(y1[1::4, 1::4], z2, 1).flags.c_contiguous
 
 
 def test_passthrough_arguments(doc):
-    from pybind11_tests import vec_passthrough, NonPODClass
-
-    assert doc(vec_passthrough) == (
-        "vec_passthrough("
-        "arg0: float, arg1: numpy.ndarray[float64], arg2: numpy.ndarray[float64], "
-        "arg3: numpy.ndarray[int32], arg4: int, arg5: m.NonPODClass, arg6: numpy.ndarray[float64]"
-        ") -> object")
+    assert doc(m.vec_passthrough) == (
+        "vec_passthrough(" + ", ".join([
+            "arg0: float",
+            "arg1: numpy.ndarray[float64]",
+            "arg2: numpy.ndarray[float64]",
+            "arg3: numpy.ndarray[int32]",
+            "arg4: int",
+            "arg5: m.numpy_vectorize.NonPODClass",
+            "arg6: numpy.ndarray[float64]"]) + ") -> object")
 
     b = np.array([[10, 20, 30]], dtype='float64')
     c = np.array([100, 200])  # NOT a vectorized argument
     d = np.array([[1000], [2000], [3000]], dtype='int')
     g = np.array([[1000000, 2000000, 3000000]], dtype='int')  # requires casting
     assert np.all(
-        vec_passthrough(1, b, c, d, 10000, NonPODClass(100000), g) ==
+        m.vec_passthrough(1, b, c, d, 10000, m.NonPODClass(100000), g) ==
         np.array([[1111111, 2111121, 3111131],
                   [1112111, 2112121, 3112131],
                   [1113111, 2113121, 3113131]]))
 
 
 def test_method_vectorization():
-    from pybind11_tests import VectorizeTestClass
-
-    o = VectorizeTestClass(3)
+    o = m.VectorizeTestClass(3)
     x = np.array([1, 2], dtype='int')
     y = np.array([[10], [20]], dtype='float32')
     assert np.all(o.method(x, y) == [[14, 15], [24, 25]])
 
 
 def test_array_collapse():
-    from pybind11_tests import vectorized_func
-
-    assert not isinstance(vectorized_func(1, 2, 3), np.ndarray)
-    assert not isinstance(vectorized_func(np.array(1), 2, 3), np.ndarray)
-    z = vectorized_func([1], 2, 3)
+    assert not isinstance(m.vectorized_func(1, 2, 3), np.ndarray)
+    assert not isinstance(m.vectorized_func(np.array(1), 2, 3), np.ndarray)
+    z = m.vectorized_func([1], 2, 3)
     assert isinstance(z, np.ndarray)
     assert z.shape == (1, )
-    z = vectorized_func(1, [[[2]]], 3)
+    z = m.vectorized_func(1, [[[2]]], 3)
     assert isinstance(z, np.ndarray)
     assert z.shape == (1, 1, 1)