test: improve coverage of fs internal utils

Refs: https://coverage.nodejs.org/coverage-910efc2d9a69ac32/lib/internal/fs/utils.js.html#L634

PR-URL: https://github.com/nodejs/node/pull/38746


Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>