test: improve assertions in pummel/test-timers

* Timers should not fire early. Check for that.
* Allow the wiggle-room to increase on subsequent iterations of
  intervals.

PR-URL: https://github.com/nodejs/node/pull/35216


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Andrey Pechkurov <apechkurov@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>