test: refactor test-fs-watchfile.js

The test no longer waits about 5 seconds between callback invocations.

It now writes to the tmp directory rather than the fixtures directory.

Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/2393