GeezerVison

…6434)

Fix some test_multiprocessing flakiness.

Potentially introduced by python#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 ea1803e)

Co-authored-by: Gregory P. Smith <[email protected]>