test: fix test-worker-memory.js for large cpu #s

This test consistently failed on a system with a
large number of cores (~120). Cap the number of
concurrent workers so we'll stay consistently within
the "slack" allowed with respect to rss.

PR-URL: https://github.com/nodejs/node/pull/27090


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>