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_operator_overloading.py

@@ -1,12 +1,11 @@
 import pytest
+from pybind11_tests import operators as m
 from pybind11_tests import ConstructorStats
 
 
 def test_operator_overloading():
-    from pybind11_tests.operators import Vector2, Vector
-
-    v1 = Vector2(1, 2)
-    v2 = Vector(3, -1)
+    v1 = m.Vector2(1, 2)
+    v2 = m.Vector(3, -1)
     assert str(v1) == "[1.000000, 2.000000]"
     assert str(v2) == "[3.000000, -1.000000]"
 
@@ -36,7 +35,7 @@ def test_operator_overloading():
     v2 /= v1
     assert str(v2) == "[2.000000, 8.000000]"
 
-    cstats = ConstructorStats.get(Vector2)
+    cstats = ConstructorStats.get(m.Vector2)
     assert cstats.alive() == 2
     del v1
     assert cstats.alive() == 1
@@ -59,9 +58,8 @@ def test_operator_overloading():
 
 def test_operators_notimplemented():
     """#393: need to return NotSupported to ensure correct arithmetic operator behavior"""
-    from pybind11_tests.operators import C1, C2
 
-    c1, c2 = C1(), C2()
+    c1, c2 = m.C1(), m.C2()
     assert c1 + c1 == 11
     assert c2 + c2 == 22
     assert c2 + c1 == 21
@@ -70,24 +68,23 @@ def test_operators_notimplemented():
 
 def test_nested():
     """#328: first member in a class can't be used in operators"""
-    from pybind11_tests.operators import NestA, NestB, NestC, get_NestA, get_NestB, get_NestC
 
-    a = NestA()
-    b = NestB()
-    c = NestC()
+    a = m.NestA()
+    b = m.NestB()
+    c = m.NestC()
 
     a += 10
-    assert get_NestA(a) == 13
+    assert m.get_NestA(a) == 13
     b.a += 100
-    assert get_NestA(b.a) == 103
+    assert m.get_NestA(b.a) == 103
     c.b.a += 1000
-    assert get_NestA(c.b.a) == 1003
+    assert m.get_NestA(c.b.a) == 1003
     b -= 1
-    assert get_NestB(b) == 3
+    assert m.get_NestB(b) == 3
     c.b -= 3
-    assert get_NestB(c.b) == 1
+    assert m.get_NestB(c.b) == 1
     c *= 7
-    assert get_NestC(c) == 35
+    assert m.get_NestC(c) == 35
 
     abase = a.as_base()
     assert abase.value == -2