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.
diff --git a/tests/test_virtual_functions.py b/tests/test_virtual_functions.py
index 7d1698d..138be63 100644
--- a/tests/test_virtual_functions.py
+++ b/tests/test_virtual_functions.py
@@ -149,7 +149,92 @@
     """
 
 
-def test_inheriting_repeat():
+# PyPy: Reference count > 1 causes call with noncopyable instance
+# to fail in ncv1.print_nc()
+@pytest.unsupported_on_pypy
+@pytest.mark.skipif(not hasattr(m, "NCVirt"), reason="NCVirt test broken on ICPC")
+def test_move_support():
+    class NCVirtExt(m.NCVirt):
+        def get_noncopyable(self, a, b):
+            # Constructs and returns a new instance:
+            nc = m.NonCopyable(a * a, b * b)
+            return nc
+
+        def get_movable(self, a, b):
+            # Return a referenced copy
+            self.movable = m.Movable(a, b)
+            return self.movable
+
+    class NCVirtExt2(m.NCVirt):
+        def get_noncopyable(self, a, b):
+            # Keep a reference: this is going to throw an exception
+            self.nc = m.NonCopyable(a, b)
+            return self.nc
+
+        def get_movable(self, a, b):
+            # Return a new instance without storing it
+            return m.Movable(a, b)
+
+    ncv1 = NCVirtExt()
+    assert ncv1.print_nc(2, 3) == "36"
+    assert ncv1.print_movable(4, 5) == "9"
+    ncv2 = NCVirtExt2()
+    assert ncv2.print_movable(7, 7) == "14"
+    # Don't check the exception message here because it differs under debug/non-debug mode
+    with pytest.raises(RuntimeError):
+        ncv2.print_nc(9, 9)
+
+    nc_stats = ConstructorStats.get(m.NonCopyable)
+    mv_stats = ConstructorStats.get(m.Movable)
+    assert nc_stats.alive() == 1
+    assert mv_stats.alive() == 1
+    del ncv1, ncv2
+    assert nc_stats.alive() == 0
+    assert mv_stats.alive() == 0
+    assert nc_stats.values() == ['4', '9', '9', '9']
+    assert mv_stats.values() == ['4', '5', '7', '7']
+    assert nc_stats.copy_constructions == 0
+    assert mv_stats.copy_constructions == 1
+    assert nc_stats.move_constructions >= 0
+    assert mv_stats.move_constructions >= 0
+
+
+def test_dispatch_issue(msg):
+    """#159: virtual function dispatch has problems with similar-named functions"""
+    class PyClass1(m.DispatchIssue):
+        def dispatch(self):
+            return "Yay.."
+
+    class PyClass2(m.DispatchIssue):
+        def dispatch(self):
+            with pytest.raises(RuntimeError) as excinfo:
+                super(PyClass2, self).dispatch()
+            assert msg(excinfo.value) == 'Tried to call pure virtual function "Base::dispatch"'
+
+            p = PyClass1()
+            return m.dispatch_issue_go(p)
+
+    b = PyClass2()
+    assert m.dispatch_issue_go(b) == "Yay.."
+
+
+def test_override_ref():
+    """#392/397: overridding reference-returning functions"""
+    o = m.OverrideTest("asdf")
+
+    # Not allowed (see associated .cpp comment)
+    # i = o.str_ref()
+    # assert o.str_ref() == "asdf"
+    assert o.str_value() == "asdf"
+
+    assert o.A_value().value == "hi"
+    a = o.A_ref()
+    assert a.value == "hi"
+    a.value = "bye"
+    assert a.value == "bye"
+
+
+def test_inherited_virtuals():
     class AR(m.A_Repeat):
         def unlucky_number(self):
             return 99
@@ -276,88 +361,3 @@
     assert obj.unlucky_number() == -7
     assert obj.lucky_number() == -1.375
     assert obj.say_everything() == "BT -7"
-
-
-# PyPy: Reference count > 1 causes call with noncopyable instance
-# to fail in ncv1.print_nc()
-@pytest.unsupported_on_pypy
-@pytest.mark.skipif(not hasattr(m, "NCVirt"), reason="NCVirt test broken on ICPC")
-def test_move_support():
-    class NCVirtExt(m.NCVirt):
-        def get_noncopyable(self, a, b):
-            # Constructs and returns a new instance:
-            nc = m.NonCopyable(a * a, b * b)
-            return nc
-
-        def get_movable(self, a, b):
-            # Return a referenced copy
-            self.movable = m.Movable(a, b)
-            return self.movable
-
-    class NCVirtExt2(m.NCVirt):
-        def get_noncopyable(self, a, b):
-            # Keep a reference: this is going to throw an exception
-            self.nc = m.NonCopyable(a, b)
-            return self.nc
-
-        def get_movable(self, a, b):
-            # Return a new instance without storing it
-            return m.Movable(a, b)
-
-    ncv1 = NCVirtExt()
-    assert ncv1.print_nc(2, 3) == "36"
-    assert ncv1.print_movable(4, 5) == "9"
-    ncv2 = NCVirtExt2()
-    assert ncv2.print_movable(7, 7) == "14"
-    # Don't check the exception message here because it differs under debug/non-debug mode
-    with pytest.raises(RuntimeError):
-        ncv2.print_nc(9, 9)
-
-    nc_stats = ConstructorStats.get(m.NonCopyable)
-    mv_stats = ConstructorStats.get(m.Movable)
-    assert nc_stats.alive() == 1
-    assert mv_stats.alive() == 1
-    del ncv1, ncv2
-    assert nc_stats.alive() == 0
-    assert mv_stats.alive() == 0
-    assert nc_stats.values() == ['4', '9', '9', '9']
-    assert mv_stats.values() == ['4', '5', '7', '7']
-    assert nc_stats.copy_constructions == 0
-    assert mv_stats.copy_constructions == 1
-    assert nc_stats.move_constructions >= 0
-    assert mv_stats.move_constructions >= 0
-
-
-def test_dispatch_issue(msg):
-    """#159: virtual function dispatch has problems with similar-named functions"""
-    class PyClass1(m.DispatchIssue):
-        def dispatch(self):
-            return "Yay.."
-
-    class PyClass2(m.DispatchIssue):
-        def dispatch(self):
-            with pytest.raises(RuntimeError) as excinfo:
-                super(PyClass2, self).dispatch()
-            assert msg(excinfo.value) == 'Tried to call pure virtual function "Base::dispatch"'
-
-            p = PyClass1()
-            return m.dispatch_issue_go(p)
-
-    b = PyClass2()
-    assert m.dispatch_issue_go(b) == "Yay.."
-
-
-def test_override_ref():
-    """#392/397: overridding reference-returning functions"""
-    o = m.OverrideTest("asdf")
-
-    # Not allowed (see associated .cpp comment)
-    # i = o.str_ref()
-    # assert o.str_ref() == "asdf"
-    assert o.str_value() == "asdf"
-
-    assert o.A_value().value == "hi"
-    a = o.A_ref()
-    assert a.value == "hi"
-    a.value = "bye"
-    assert a.value == "bye"