Just launch another instance of the game and use that, or invest in something like QEMU and Virtual PC.

The way I used to set it up was with three instances on my computer and three instances on my brother's. I'm doing this because the actual game server instance needs to be able to handle all the traffic between 5 clients. The problem is that with any more than three instances running on my computer, you can't tell the difference between the computer running slow and the network code waiting a quarter of a second to do its thing...

Basically I was using his computer for running the server and two clients, and I was running three clients. The server would sometimes get caught with multiple requests and goof off for .25-1 second... that was why I needed his computer.

For the everyday testing between the normal server and one client, I just run it localhost. But it was the stress testing of the server that I was going for...