https://github.com/vito/houdini
There's a blog post by someone else at Pivotal, although the details are probably a bit out of date by now:
https://blog.pivotal.io/labs/tech-talks/on-device-open-sourc...
At Pivotal Toronto we're running iOS and Android builds using a mac mini as an external worker. What's been a great built in feature is using the pool resource to "check-out" a device and lock it from other tests while a suite is running.
Also, if you have some free time I would love to chat about your experience so far -- I work about 15 minutes away from Pivotal's Toronto office. My email address is listed in my profile.
I find these approaches better than risking test pollution, especially when it comes to building VMs and spinning up desktop apps.