test: make sure weak references are not GCed too early

This fixes flaky `util.inspect` tests by making sure that the
inspected WeakMap and WeakSet only contains objects that still have
other hard references. Otherwise the garbage collection could collect
these objects to early.

PR-URL: https://github.com/nodejs/node/pull/27482


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>