test: refactor the code in test-fs-watch.js

* used let and const instead of var
* used assert.strictEqual instead assert.equal

PR-URL: https://github.com/nodejs/node/pull/10357


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Julian Duque <julianduquej@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>