test: wrap countdown callback in common.mustCall

This adds a implicit common.mustCall to the callback provided to
the countdown.

PR-URL: https://github.com/nodejs/node/pull/18506


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>