[3.12] gh-88118: Fix some test_multiprocessing flakiness. (GH-116434) (GH-116440)

Fix some test_multiprocessing flakiness.

Potentially introduced by https://github.com/python/cpython/pull/25845

not joining that thread likely leads to recently observed "environment
changed" logically passing but overall failing tests seen on some
buildbots similar to:

```
1 test altered the execution environment (env changed):
    test.test_multiprocessing_fork.test_processes

2 re-run tests:
    test.test_multiprocessing_fork.test_processes
    test.test_multiprocessing_forkserver.test_processes
```
(cherry picked from commit ea1803e608a7aaf9cf2c07e510d8540d46d3b9ad)

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