[3.12] gh-130736: Fix asyncio test_shutdown_default_executor_timeout() (GH-130800) (#130826)

gh-130736: Fix asyncio test_shutdown_default_executor_timeout() (GH-130800)

Replace time.sleep() with threading.Event.
(cherry picked from commit 6c48ed7d62c6ca0eb24935b0e612f9e4a1a3b1bc)

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