Make it possible to run gitlab-ce tests with a local modified copy of Gitaly
At the moment the gitlab-ce test suite will always download its own copy of Gitaly from the internet, and compile and boot that copy when running tests. This makes development hard because you need to make changes in gitaly and push them to a branch each time you want to re-run gitlab-ce tests.
We should add a mechanism where it is possible to run local tests with a local, modified copy of Gitaly instead of always downloading. This will make the feedback cycle shorter.
CI should still use the 'download Gitaly from the internet' approach.