test: refactor test-fs-watch-non-recursive

The test was duplicating some functionality in the `tmpdir` module.

PR-URL: https://github.com/nodejs/node/pull/25386


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>