test: move test-worker-eventlooputil to sequential

The test is not nearly as unreliable as it used to be but we're still
seeing failures around the timing checks that will definitely be
affected by other tests running in other processes. So move it to
sequential.

Refs: https://github.com/nodejs/node/pull/35961#issuecomment-722222696

PR-URL: https://github.com/nodejs/node/pull/35996


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>