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.cpp

@@ -16,22 +16,11 @@ public:
     Vector2(float x, float y) : x(x), y(y) { print_created(this, toString()); }
     Vector2(const Vector2 &v) : x(v.x), y(v.y) { print_copy_created(this); }
     Vector2(Vector2 &&v) : x(v.x), y(v.y) { print_move_created(this); v.x = v.y = 0; }
+    Vector2 &operator=(const Vector2 &v) { x = v.x; y = v.y; print_copy_assigned(this); return *this; }
+    Vector2 &operator=(Vector2 &&v) { x = v.x; y = v.y; v.x = v.y = 0; print_move_assigned(this); return *this; }
     ~Vector2() { print_destroyed(this); }
 
-    std::string toString() const {
-        return "[" + std::to_string(x) + ", " + std::to_string(y) + "]";
-    }
-
-    void operator=(const Vector2 &v) {
-        print_copy_assigned(this);
-        x = v.x;
-        y = v.y;
-    }
-
-    void operator=(Vector2 &&v) {
-        print_move_assigned(this);
-        x = v.x; y = v.y; v.x = v.y = 0;
-    }
+    std::string toString() const { return "[" + std::to_string(x) + ", " + std::to_string(y) + "]"; }
 
     Vector2 operator+(const Vector2 &v) const { return Vector2(x + v.x, y + v.y); }
     Vector2 operator-(const Vector2 &v) const { return Vector2(x - v.x, y - v.y); }
@@ -64,30 +53,9 @@ int operator+(const C2 &, const C2 &) { return 22; }
 int operator+(const C2 &, const C1 &) { return 21; }
 int operator+(const C1 &, const C2 &) { return 12; }
 
-struct NestABase {
-    int value = -2;
-};
-
-struct NestA : NestABase {
-    int value = 3;
-    NestA& operator+=(int i) { value += i; return *this; }
-};
-
-struct NestB {
-    NestA a;
-    int value = 4;
-    NestB& operator-=(int i) { value -= i; return *this; }
-};
-
-struct NestC {
-    NestB b;
-    int value = 5;
-    NestC& operator*=(int i) { value *= i; return *this; }
-};
-
-test_initializer operator_overloading([](py::module &pm) {
-    auto m = pm.def_submodule("operators");
+TEST_SUBMODULE(operators, m) {
 
+    // test_operator_overloading
     py::class_<Vector2>(m, "Vector2")
         .def(py::init<float, float>())
         .def(py::self + py::self)
@@ -113,6 +81,7 @@ test_initializer operator_overloading([](py::module &pm) {
 
     m.attr("Vector") = m.attr("Vector2");
 
+    // test_operators_notimplemented
     // #393: need to return NotSupported to ensure correct arithmetic operator behavior
     py::class_<C1>(m, "C1")
         .def(py::init<>())
@@ -124,29 +93,44 @@ test_initializer operator_overloading([](py::module &pm) {
         .def("__add__", [](const C2& c2, const C1& c1) { return c2 + c1; })
         .def("__radd__", [](const C2& c2, const C1& c1) { return c1 + c2; });
 
+    // test_nested
     // #328: first member in a class can't be used in operators
+    struct NestABase { int value = -2; };
     py::class_<NestABase>(m, "NestABase")
         .def(py::init<>())
         .def_readwrite("value", &NestABase::value);
 
+    struct NestA : NestABase {
+        int value = 3;
+        NestA& operator+=(int i) { value += i; return *this; }
+    };
     py::class_<NestA>(m, "NestA")
         .def(py::init<>())
         .def(py::self += int())
         .def("as_base", [](NestA &a) -> NestABase& {
             return (NestABase&) a;
         }, py::return_value_policy::reference_internal);
+    m.def("get_NestA", [](const NestA &a) { return a.value; });
 
+    struct NestB {
+        NestA a;
+        int value = 4;
+        NestB& operator-=(int i) { value -= i; return *this; }
+    };
     py::class_<NestB>(m, "NestB")
         .def(py::init<>())
         .def(py::self -= int())
         .def_readwrite("a", &NestB::a);
+    m.def("get_NestB", [](const NestB &b) { return b.value; });
 
+    struct NestC {
+        NestB b;
+        int value = 5;
+        NestC& operator*=(int i) { value *= i; return *this; }
+    };
     py::class_<NestC>(m, "NestC")
         .def(py::init<>())
         .def(py::self *= int())
         .def_readwrite("b", &NestC::b);
-
-    m.def("get_NestA", [](const NestA &a) { return a.value; });
-    m.def("get_NestB", [](const NestB &b) { return b.value; });
     m.def("get_NestC", [](const NestC &c) { return c.value; });
-});
+}