Replace testhelper.NewServerWithAuth with testserver.RunGitaly

The old testhelper.NewServerWithAuth function uses non-production code to set up gRPC server. By replacing it with testserver.RunGitaly we try to increase code re-usability and test with production code. The setup also less error prone and already used in a lot of places.

Unfortunately we should move transaction package to transaction_test package to break circular dependencies between the packages.

The change also removes some other unused code.