test: improve test-fs-access

* use const and let instead of var
* use common.mustCall to control functions execution
* use assert.ifError instead of assert.strictEqual for errors
* use arrow functions

PR-URL: https://github.com/nodejs/node/pull/10542


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