Add a pipeline step to run Gitaly tests through Praefect servers

based on gitlab!18371 (comment 238395747), we want to have gitaly run its tests through praefect to ensure that praefect behaves transparently.

• in Gitaly's testhelper.NewTestGrpcServer, check an environment variable GITALY_TESTING_PRAEFECT_BIN. If empty, do what we do today: return a unix socket that goes straight to a gitaly grpc test server. If not empty, spawn > a praefect process, and return a unix socket that goes to that praefect process.
• in Gitaly CI, add an extra job where we run make test with GITALY_TESTING_PRAEFECT_BIN set.