test,fs: test fs.watch for `filename`

PR-URL: https://github.com/nodejs/node/pull/13411
Refs: https://github.com/nodejs/node/pull/13385
Refs: https://github.com/nodejs/node/issues/13248
Refs: https://github.com/nodejs/node/issues/13377


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>