benchmark: use unique filenames in fs benchmarks

Use a unique file name for each benchmark. Running benchmarks
simultaneously may be a bit of an unusual use case, but there are use
cases, such as stress testing `test/parallel/test-benchmark-fs.js`.

PR-URL: https://github.com/nodejs/node/pull/16776


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>