test: clean up net server try ports test

* Replace var's with const and let
* Replace boolean flags with common.mustCall()
* Using stricter comparisons
* Fixed typo in comment

PR-URL: https://github.com/nodejs/node/pull/8458


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>