Honestly, why not just involve a few friends and get them to help with the testing? That would also give you the benefit of seeing how the server performs under "real-world" network conditions, rather than just under ideal settings (a LAN).

I'm sure the "pros" have some tools that help them with automated testing, but I'd expect most of them to probably be either custom-made or big $$$ commercial software. There are certainly things you could do yourself like writing a non-graphical client, but is it really going to be worth the time and effort it will take?