bpo-44287: asyncio test_popen() uses longer timeout (GH-26832)

Fix asyncio test_popen() of test_windows_utils by using a longer
timeout. Use military grade battle-tested test.support.SHORT_TIMEOUT
timeout rather than a hardcoded timeout of 10 seconds: it's 30
seconds by default, but it is made longer on slow buildbots.

WaitForMultipleObjects() timeout argument is in milliseconds.
(cherry picked from commit be1cb3214d09d4bf0288bc45f3c1f167f67e4514)

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