Make it easy to have Gitaly in tests serving multiple working storages

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Right now our Gitaly config under the test environment has only two storages (default and broken). Some specs may need to test interactions between two different storages (e.g. forking between storages), but it's not easy to have a new storage without breaking some specs as they expect only two. I think we should have a quick way to spawn another Gitaly instance with custom config to test such interactions.