test: fix flaky test-fs-watchfile on macOS

On macOS, a watcher created with fs.watch() does not necessarily
start receiving events immediately. So it can miss a change by
fs.writefile() if it comes very soon after the watcher is created. Fix
test flakiness caused by this by using `setInterval()` to repeat the
write action.

PR-URL: https://github.com/nodejs/node/pull/13252
Fixes: https://github.com/nodejs/node/issues/13248


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>