test: improve assertion message for test-vm-memleak

The test is unreliable in CI. It might be helpful to get an exact read
of `rss` so this adds that to the assertion message.

PR-URL: https://github.com/nodejs/node/pull/37034


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>