Fix Gitaly address for tests

Previously only the Gitaly process was started in tests, but the Gitaly address was configured to use praefect.socket because the GDK may have been configured to use Praefect. However, the change in gitlab!26323 (merged) caused specs to fail because the test environment expects both a Gitaly and Praefect address to be available on distinct sockets (e.g. tmp/tests/gitaly.socket and tmp/tests/praefect.socket). Since gitaly.socket wasn't available, the test would fail to start.

Now, the GDK should always be using gitaly.socket for tests, so we need to adjust this config accordingly.

Closes gitlab#212509 (closed)