test: improve coverage of lib/fs.js

PR-URL: https://github.com/nodejs/node/pull/38604
Refs: https://coverage.nodejs.org/coverage-29f1b609ba5d12d3/lib/fs.js.html#L2045


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>