Refactor praefect server tests

This change refactors some of the test helpers in internal/praefect so that they enable Gitaly integration style tests. This way we can exercise the Praefect server in a more end-to-end style that ensures all moving parts in the server are working in concert.

Helper functions were created/updated to streamline the writing of these tests.

The mock service used by Praefect tests now uses our Gitaly HA annotations. The mock server implementation (mockSvc) was updated to make it convenient to partially update RPC implementations via callbacks.

The unit test TestServerSimpleUnaryUnary was rewritten and renamed to TestServerRouteServerAccessor to reflect the type of routing we expect for this RPC annotation.

No implementation code was changed in this MR.