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_chrono.py

@@ -1,11 +1,11 @@
+from pybind11_tests import chrono as m
+import datetime
 
 
 def test_chrono_system_clock():
-    from pybind11_tests import test_chrono1
-    import datetime
 
     # Get the time from both c++ and datetime
-    date1 = test_chrono1()
+    date1 = m.test_chrono1()
     date2 = datetime.datetime.today()
 
     # The returned value should be a datetime
@@ -25,13 +25,10 @@ def test_chrono_system_clock():
 
 
 def test_chrono_system_clock_roundtrip():
-    from pybind11_tests import test_chrono2
-    import datetime
-
     date1 = datetime.datetime.today()
 
     # Roundtrip the time
-    date2 = test_chrono2(date1)
+    date2 = m.test_chrono2(date1)
 
     # The returned value should be a datetime
     assert isinstance(date2, datetime.datetime)
@@ -44,8 +41,6 @@ def test_chrono_system_clock_roundtrip():
 
 
 def test_chrono_duration_roundtrip():
-    from pybind11_tests import test_chrono3
-    import datetime
 
     # Get the difference between two times (a timedelta)
     date1 = datetime.datetime.today()
@@ -55,7 +50,7 @@ def test_chrono_duration_roundtrip():
     # Make sure this is a timedelta
     assert isinstance(diff, datetime.timedelta)
 
-    cpp_diff = test_chrono3(diff)
+    cpp_diff = m.test_chrono3(diff)
 
     assert cpp_diff.days == diff.days
     assert cpp_diff.seconds == diff.seconds
@@ -63,14 +58,12 @@ def test_chrono_duration_roundtrip():
 
 
 def test_chrono_duration_subtraction_equivalence():
-    from pybind11_tests import test_chrono4
-    import datetime
 
     date1 = datetime.datetime.today()
     date2 = datetime.datetime.today()
 
     diff = date2 - date1
-    cpp_diff = test_chrono4(date2, date1)
+    cpp_diff = m.test_chrono4(date2, date1)
 
     assert cpp_diff.days == diff.days
     assert cpp_diff.seconds == diff.seconds
@@ -78,22 +71,13 @@ def test_chrono_duration_subtraction_equivalence():
 
 
 def test_chrono_steady_clock():
-    from pybind11_tests import test_chrono5
-    import datetime
-
-    time1 = test_chrono5()
-    time2 = test_chrono5()
-
+    time1 = m.test_chrono5()
     assert isinstance(time1, datetime.timedelta)
-    assert isinstance(time2, datetime.timedelta)
 
 
 def test_chrono_steady_clock_roundtrip():
-    from pybind11_tests import test_chrono6
-    import datetime
-
     time1 = datetime.timedelta(days=10, seconds=10, microseconds=100)
-    time2 = test_chrono6(time1)
+    time2 = m.test_chrono6(time1)
 
     assert isinstance(time2, datetime.timedelta)
 
@@ -104,17 +88,14 @@ def test_chrono_steady_clock_roundtrip():
 
 
 def test_floating_point_duration():
-    from pybind11_tests import test_chrono7, test_chrono_float_diff
-    import datetime
-
-    # Test using 35.525123 seconds as an example floating point number in seconds
-    time = test_chrono7(35.525123)
+    # Test using a floating point number in seconds
+    time = m.test_chrono7(35.525123)
 
     assert isinstance(time, datetime.timedelta)
 
     assert time.seconds == 35
     assert 525122 <= time.microseconds <= 525123
 
-    diff = test_chrono_float_diff(43.789012, 1.123456)
+    diff = m.test_chrono_float_diff(43.789012, 1.123456)
     assert diff.seconds == 42
     assert 665556 <= diff.microseconds <= 665557