test: refactor the code in test-fs-chmod

* use const and let instead of var
* use common.mustCall to control functions executions
* use assert.strictEqual instead of assert.equal
* use assert.ifError to handle errors
* use arrow functions
* remove unnecessary variables

PR-URL: https://github.com/nodejs/node/pull/10440


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