More tests on Forked Repos (gitaly object pools)

An common source of issues that can lead to bugs in Gitaly is in the area of the object pool. The object pool is typically used in the forking workflow, as it provides a mechanism to share objects between repositories, without needing to duplicate them, hence saving disk space.

However, due to the complexity of this mechanism, we have on occasion found bugs in this area.
The proposal here is to investigate how we can effectively harden this area of the codebase by expanding test coverage both direclty, and indirectly against forked repos in the whole.

See: https://docs.gitlab.com/ee/architecture/blueprints/object_pools/ for future plans for Object Pools.