gh-89898: Fix test_threading.test_args_argument() (GH-92885)

Join the thread to not leak threads running in the background to the
next test.

Fix the following warning on the "AMD64 FreeBSD Shared 3.11"
buildbot:

test_args_argument (test.test_threading.ThreadTests.test_args_argument) ...
Warning -- threading_cleanup() failed to cleanup 1 threads (count: 1, dangling: 2)
Warning -- Dangling thread: <_MainThread(MainThread, started 35026161664)>
Warning -- Dangling thread: <Thread(Thread-134 (<lambda>), started 35314998016)>
ok
(cherry picked from commit 970efae274538f9910e8f9b31bc890f1b7666b0f)

Co-authored-by: Victor Stinner <vstinner@python.org>