test: improve test-fs-write-file-sync

* use let and const instead of var
* use assert.strictEqual instead of assert.equal
* swap assertions arguments to match the standard

PR-URL: https://github.com/nodejs/node/pull/10624


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