test: improve fs coverage

PR-URL: https://github.com/nodejs/node/pull/38517
Refs: https://coverage.nodejs.org/coverage-68e6673224365120/lib/fs.js.html#L330


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