[3.12] gh-127258: Fix asyncio test_staggered_race_with_eager_tasks() (GH-127358) (#127402)

gh-127258: Fix asyncio test_staggered_race_with_eager_tasks() (GH-127358)

Replace the sleep(2) with a task which is blocked forever.
(cherry picked from commit bfabf96b50b7d6a9c15b298a86ba3633b05a1fd7)

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