test: refactor test-http-server-keep-alive-timeout

Make the same reliability changes that were applied to the https test in
ce5745bf.

Refs: https://github.com/nodejs/node/pull/13312
PR-URL: https://github.com/nodejs/node/pull/13448


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>