[3.14] gh-133744: Fix multiprocessing interrupt test: add an event (GH-133746) (#133916)

gh-133744: Fix multiprocessing interrupt test: add an event (GH-133746)

Add an event to synchronize the parent process with the child
process: wait until the child process starts sleeping.
(cherry picked from commit c2989b7070b18c0b7c51521fed8bc11c159ea5b8)

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