test: move test-timers-throw-reschedule to sequential

This also sets the test as flaky

PR-URL: https://github.com/nodejs/node/pull/22379
Refs: https://github.com/nodejs/node/issues/21188


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>