gh-88118: Fix some test_multiprocessing flakiness. (#116434)

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
```