testserver: Use server factory to construct server

With commit 4971502f (server: Use injected Cache as cache invalidator, 2021-05-12), a segfault was introduced into Gitaly because the injected cache wasn't actually set up as a field for the server factory. While the segfault was easy to fix, it shows that we are lacking test coverage of production code. This lacking coverage is caused by the fact that we never construct servers for our tests via the server factory, but in production we do.

Let's plug this blind spot by changing the testserver code to use the server factory.