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 <[email protected]>
Reviewed-By: Gus Caplan <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>
Reviewed-By: Sakthipriyan Vairamani <[email protected]>

Author: Trott