test: refactor test-fs-write-file-sync.js

This commit reduces shared state by introducing scopes and
block scoped variables. It also makes the monkey patching used
by the test more robust.

PR-URL: https://github.com/nodejs/node/pull/24834


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>