test: refactor pummel/test-net-pingpong

* Use port 0 instead of `common.PORT`.
* Use `//` for comments, capitalize comments, and add punctuation.

PR-URL: https://github.com/nodejs/node/pull/25485


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>