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).