Sorry for the slow reply I've only just seen this. There isn't a way to launch an appx using PowerShell (actually you'll be able to find a script [metro-launch] on the web that can). Continuous integration here isn't really a problem since sideloading is a way to deploy the app for the end user. I'm not aware of any automated test tools yet for Windows 8 apps either.
Hope that helps, ping me back if not or I can help more.