test: make test-fs-watchfile reliable

Omitting AIX from `fs.watch()` portion of this test. It works
on AIX, but not reliably.

PR-URL: https://github.com/nodejs/node/pull/13385
Fixes: https://github.com/nodejs/node/issues/13377


Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>