test: fix flaky timers-block-eventloop test

Due to extensive reliance on timings and the fs module, this test
is currently inherently flaky. Refactor it to simply use setImmediate
and only one busy loop.

PR-URL: https://github.com/nodejs/node/pull/18567


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>