test: added fs benchmark test

Modified fs benchmarks to use more unique args to avoid collision.

PR-URL: https://github.com/nodejs/node/pull/16049


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>