Add praefect as a transparent pass through for tests

fixes: #2122 (closed)

This MR adds a new type testhelper.TestServer that wraps a *grpc.Server, and handles creating sockets and listeners. In the gitaly case, it just creates a socket file and listens to it. In the case we want praefect, it will spawn a praefect process that has one virtual storage with one gitaly node behind it.

It also adds a pipeline step to run the suite through CI with GITALY_TEST_PRAEFECT_BIN=1.

Another MR will be necessary to refactor all tests to use the testhelper.TestServer helper, as this one only contains changes for tests in the blob service.