test: refactor pummel/test-keep-alive

* Reduce concurrent and duration options by half so as to avoid
  interference with other tests. (Excessive TCP activity in this test
  resulted in throttling that caused subsequent tests to fail on my
  local setup.)
* Use an OS-provided port rather than `common.PORT`. This possibly
  reduces side-effects on other tests (that may also be using
  `common.PORT`).
* Add punctuation in comments.

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>