test: improve test-gc-http-client-onerror

test-gc-http-client-onerror 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: #23090
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

Author: Trott