test: move some gc tests back to parallel/, unmark flaky

These should no longer be flaky after the libuv update.

Refs: https://github.com/nodejs/node/pull/23336

PR-URL: https://github.com/nodejs/node/pull/23356


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>