test: improve reliability of test-gc-http-client

test-gc-http-client is resource-intensive. It times out a lot on CI.
Move to sequential.

Approved for fast-tracking by:
Anna Henningsen <anna@addaleax.net>
Daniel Bevenius <daniel.bevenius@gmail.com>

PR-URL: #23087
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

Author: Trott