Replace usages of the testhelper.NewServer for gitaly services

Gitaly service setup done with testhelper.NewServer uses a lot of additional code that is not part of the production. It only creates a gRPC server and you need to manually register all required services on it. Including creation of all the dependencies required for those service. The new testserver.RunGitalyServer function does most of those operations for you. It uses mostly production code to setup the server. It also provides you with meaningful values for the dependencies you can use for services registration.

The repository package is not covered as that replacement will be done in terms of !3326 (merged)