Adding dedicated test_const_name. (#3578)

* Adding dedicated test_const_name.

Also exercises pybind11::detail::_ backward compatibility.

See also: PR #3423

* Backing out tests involving int_to_str (requires C++17 or higher).

* Suppressing clang-tidy errors.

* Disabling test_const_name for MSVC 2015 due to bizarre failures.

* Stacking @pytest.mark.parametrize (thanks to @skylion007 for pointing out).

tests/CMakeLists.txt

@@ -101,6 +101,7 @@ set(PYBIND11_TEST_FILES
     test_callbacks.cpp
     test_chrono.cpp
     test_class.cpp
+    test_const_name.cpp
     test_constants_and_functions.cpp
     test_copy_move.cpp
     test_custom_type_casters.cpp