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

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

Fixes: https://github.com/nodejs/node/issues/29344

Collaborators, please 👍 to fast-track.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines