test: call gc() explicitly to avoid OOM

PR-URL: https://github.com/nodejs/node/pull/22301
Refs: https://github.com/nodejs/reliability/issues/12
Refs: https://github.com/nodejs/node/issues/16354


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>