test: improve code in test-fs-open.js

* use const and let instead of var
* use assert.strictEqual instead of assert.equal
* use assert.strictEqual instead of assert.ok
* use assert.ifError

PR-URL: https://github.com/nodejs/node/pull/10312


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>