Fragile test: TestSuccessfulCalculateChecksum

It seems this test assumes that the refs of gitlab-org/gitlab-test never change. That assumption is false, making this test very fragile.

https://gitlab.com/gitlab-org/gitaly/blob/b4115ab5907ea3304a387357fcb10f4e0308b516/internal/service/repository/calculate_checksum_test.go#L13-30