test: improve test-gc-http-client-timeout

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

Approved for fast-tracking by:
Anna Henningsen <[email protected]>
Daniel Bevenius <[email protected]>

PR-URL: #23088
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>

Author: Trott