Minor tweaks. (#3230)

* Minor tweaks.

* Restoring tests/pybind11_tests.h version from master, removing just the comment and empty line that was added in PR #3087; those were made obsolete by the pragma cleanup that concluded with PR #3186.

* [ci skip] Restoring tests/test_enum.py from master.

tests/pybind11_tests.h

 #pragma once
 
-// This must be kept first for MSVC 2015.
-// Do not remove the empty line between the #includes.
 #include <pybind11/pybind11.h>
-
 #include <pybind11/eval.h>
 
 #if defined(_MSC_VER) && _MSC_VER < 1910