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