test: fix flaky test-fs-promises-file-handle-close

Sometimes, the expected warnings were not emitted.
To harden the test:

- Instead of relying on a timeout that raced against the file being
  opened, wait until the file is opened and then schedule GC
  after that.
- Explicitly keep the event loop alive for one turn to make sure
  the warning is being seen.

PR-URL: https://github.com/nodejs/node/pull/31687


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>