testserver: Fix dialing Praefect server if socket does not yet exist

When running the test-with-praefect target, then we're running the Praefect executable as a transparent proxy in front of Gitaly servers. We frequently observe test failures though because we fail to establish the server's healthiness, where the error message typically indicates that the server's socket path does not exist. This may have two root causes: either we're really taking too long to spawn Praefect, or we just fail to dial Praefect correctly in case the socket really hasn't been created yet.

Plug one of these two potential races by precreating the Praefect socket path. If this doesn't fix the issue, then the only thing left we can do is to bump the three second timeout.

Part of #3835 (closed)