test: mark async-hooks/test-statwatcher as flaky

Refs: https://github.com/nodejs/node/issues/21425

PR-URL: https://github.com/nodejs/node/pull/22330


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>