Allow local tests to use a modified Gitaly

What does this MR do?

Make it possible for developers to run local tests with a locally modified version of Gitaly.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Before this change, tests could only use a version of Gitaly pushed to https://gitlab.com/gitlab-org/gitaly. Moreover on each test run we would clone this repo again.

This new mechanism is meant to speed up local development.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes gitaly#833 (closed)