investigate flaky test-fs-promises-watch on macOS

[Trott]

• Test: test-fs-promises-watch
• Platform: macOS 10.15
• Console Output:

not ok 1225 parallel/test-fs-promises-watch
  ---
  duration_ms: 120.38
  severity: fail
  exitcode: -15
  stack: |-
    timeout
  ...

• Build Links: https://ci.nodejs.org/job/node-test-commit-osx/39334/nodes=osx1015/consoleText

[Trott]

This is almost certainly a race condition whereby a watcher is returned but is not yet registering/reporting events, or something similar to that.

[Linkgoron]

I think I've managed to reproduce this a few times. At least for me, it's consistently the directory check that fails. Maybe adding watchers on directories is somehow slower, and, as you've said, it creates a race condition on MacOS. Note that the callback-based test that this is based on actually does a setInterval whereas this test does setImmediate (well, it appears that this same issue was present last time and that's why it's using setinterval, although when I tested this test locally, a ~500ms unrefed timeout was also good enough, but maybe an interval is safer).