test: improve reliability of test-gc-net-timeout

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

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

PR-URL: #23086
Fixes: #23067
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

Author: Trott