test: refactor test-net-GH-5504

* Improve comments describing test.
* Remove unnecessary `common.noop`
* Remove confusing scoping of `c` identifier

PR-URL: https://github.com/nodejs/node/pull/13025


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>