test: refactor test-watch-file.js

* use const and let instead of var
* use arrow function
* removed console.log statements

PR-URL: https://github.com/nodejs/node/pull/10679


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>