[3.12] gh-110695: test_asyncio uses 50 ms for clock resolution (GH-110952) (#110970)

gh-110695: test_asyncio uses 50 ms for clock resolution (GH-110952)

Before utils.CLOCK_RES constant was added (20 ms), test_asyncio
already used 50 ms.
(cherry picked from commit 9a9fba825f8aaee4ea9b3429875c6c6324d0dee0)

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