This merge request was originally introduced in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40026 but reverted
in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40245. We fix
the issue it caused by using `Gitlab::Redis::SharedState.url`, which
handles the differences between the GDK and CI `resque.yml`
configuration formats.

We started using Workhorse in feature tests, but no `config.toml` was
 provided. As a result, we missed a startup issue that could have been
caught by testing the config loading.

A small subset of Workhorse functionality also relies on Workhorse and
Rails sharing a Redis database. If we configure it here, we can enhance
the runner API testing in rails to ensure it behaves as expected.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/209245