test: refactor pummel/test-net-many-clients

* Use port 0 instead of `common.PORT`.
* Reduce `concurrent` from 100 to 50 and `connections_per_client` from 5
  to 3. This is to avoid side effects from other tests. Prior to this
  change, running this along with test-keep-alive would result in
  failures on my local setup, apparently due to network throttling.
* Remove unnecessary `console.log()` and improve remaining
  `console.log()` to provide clearer information.

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>