test: refactor the code of test-keep-alive.js

* use const and let instead of var
* use assert.strictEqual instead of assert.equal
* use arrow functions
* removed console.log statements

PR-URL: https://github.com/nodejs/node/pull/10684


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