test: make flaky test stricter

Make assertions in flaky test stricter to possibly
make it easier to determine the root cause.

Refs: https://github.com/nodejs/node/issues/4066

PR-URL: https://github.com/nodejs/node/pull/33539


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>