test: increase slop limit in memory leak test

It transpires that the extra bookkeeping in debug builds sometimes makes
the increase in RSS go _just_ over the 5 MB limit, by fewer than 100 kB.
Double the limit so we hopefully don't run into it any time again soon.

The memory leak it tests for was one where RSS grew by hundreds of
megabytes over the lifetime of the test; 5 vs. 10 MB is insignificant.

Fixes: https://github.com/nodejs/node/issues/21076

PR-URL: https://github.com/nodejs/node/pull/21080
Refs: https://github.com/nodejs/node/issues/21076


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>