test: unflake async-hooks/test-statwatcher

On Windows 2016 under high load further change events can be emitted
after writing the 5 bytes is reported. Updating the mtime of the file
can be reported as a separate change. This will increase the "before"
count, but not the "w1HookCount" since we removed the listener.

This makes the test keep the listeners until the end of the test.

Fixes: https://github.com/nodejs/node/issues/21425

PR-URL: https://github.com/nodejs/node/pull/32484


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>