test: harden test-gc-http-client

This reduces the total number of requests from 500 to 300 and triggers
more requests in parallel. It also moves some function creation out
and waits with the first request until the server is listening.

PR-URL: https://github.com/nodejs/node/pull/22373
Fixes: https://github.com/nodejs/node/issues/22336


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>