test: harden test-gc-http-client-timeout

* decrease number of requests 500 -> 300
* extract 'cb' to a file-local function

This should make test more reliable and less resource intensive.

PR-URL: https://github.com/nodejs/node/pull/23184
Refs: https://github.com/nodejs/node/issues/23066


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>