gh-117293: Fix race condition in run_workers.py (#117298)

The worker thread may still be alive after it enqueues it's last result,
which can lead to a delay of 30 seconds after the test finishes. This
happens much more frequently in the free-threaded build with the GIL
disabled.

This changes run_workers.py to track of live workers by enqueueing a
`WorkerExited()` instance before the worker exits.