[3.12] gh-130737: Fix multiprocessing test_notify() (GH-130797) (#130803)

gh-130737: Fix multiprocessing test_notify() (GH-130797)

Replace hardcoded delay (100 ms) with a loop awaiting until a
condition is true: replace assertReturnsIfImplemented() with
assertReachesEventually().

Use sleeping_retry() in assertReachesEventually() to tolerate slow
buildbots and raise an exception on timeout (30 seconds).
(cherry picked from commit 8a64a62002fa3cdc93cb4cfee315edb235cad8cb)

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