[3.12] gh-90876: Restore the ability to import multiprocessing when `sys.executable` is `None` (GH-106464) (#106494)

gh-90876: Restore the ability to import multiprocessing when `sys.executable` is `None` (GH-106464)

Prevent `multiprocessing.spawn` from failing to *import* in environments
where `sys.executable` is `None`.  This regressed in 3.11 with the addition
of support for path-like objects in multiprocessing.

Adds a test decorator to have tests only run when part of test_multiprocessing_spawn to `_test_multiprocessing.py` so we can start to avoid re-running the same not-global-state specific test in all 3 modes when there is no need.
(cherry picked from commit c60df361ce2d734148d503f4a711e67c110fe223)

Co-authored-by: Gregory P. Smith <greg@krypto.org>