Use Praefect instead of Gitaly in testing loop

In the test environment, we go through the trouble of spinning up Gitaly
and Praefect, only to bypass Praefect entirely and go directly to the
Gitaly socket. This renders the Praefect step useless and causes us to
miss errors in proxying.

To fix this, we now proxy all calls through Praefect and add a second
Gitaly shard.

This was discovered in
https://gitlab.com/gitlab-org/gitaly/-/issues/3379.