test: fix flaky test-http-server-keepalive-req-gc

Use `server` to keep the event loop open until the `ongc` listener runs.

PR-URL: https://github.com/nodejs/node/pull/29347
Fixes: https://github.com/nodejs/node/issues/29344


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>