gh-109566: regrtest _add_python_opts() handles KeyboardInterrupt (#110062)

In the subprocess code path, wait until the child process completes
with a timeout of EXIT_TIMEOUT seconds.

Fix create_worker_process() regression: use start_new_session=True if
USE_PROCESS_GROUP is true.

WorkerThread.wait_stopped() uses a timeout of 60 seconds, instead of
30 seconds.