test: improve test-gc-http-client-connaborted

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

PR-URL: #23091
Fixes: https://github.com/metadata
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

Author: Trott