test: improve statwatcher async_hooks test

Modify the `fs.watchFile()` async hooks test to be more
accurate; currently, it relies on undocumented methods
and the fact that they use `MakeCallback()` even though
there is always a JS stack below.

PR-URL: https://github.com/nodejs/node/pull/21244


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>