In fact, the first time I did this, VS spawned two WebDev server instances, then a third where the tests were actually run. The other two remained active when the tests ended. The one that appeared then disappeared was for the Web project (file-based with testing code), while the second one pointed the same place, and the third pointed to the production Web project (also file-based). Needless to say, it's the one that appeared then disappeared that you have to attach to, as that's the one that is acting as the environment for the tests.
Why is it that with every step forward, we take a step back?