[RUN ALL RSPEC] 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.

We also need to add a second Gitaly process since a few tests depend on testing multiple storage shards (e.g. backup and replicate, see failures in https://gitlab.com/gitlab-org/gitlab/-/pipelines/229409013). I looked into the stubbing out the backup case via Gitlab::GitalyClient.call, but you would essentially have to mock Gitaly completely to make this work. Besides, it does seem helpful to use multiple shards when testing Rails, Praefect, and Gitaly.

This was discovered in gitaly#3379 (closed)