So, I ran into the same problem with Jenkins, and I had a build step that ran "open -a simulator" before running unit tests. (I think I read somewhere that this will open the simulator so that it runs around without Jenkins headless).
I hit my head about this, so I rebooted Xcode 7.2.1 to verify that it worked, and used xcode-select to switch versions, then I switched back and saw that the problem still appeared in 7.3. I noticed that the problem in 7.3, apparently, was that several simulators opened when I played the assembly.
Finally, I removed the "open -a simulator" command and it seemed to work. I still have one jenkins build that fails to figure out when the final tests finish, but I had no problem running the tests. A string that still hangs does not replace the application delegate for an individual testing application delegate. (something like this: http://qualitycoding.org/app-delegate-for-tests/ ) So I hope this can be a problem. Also note that my projects, which only test libraries, never showed bad behavior, they could always run tests.
What so far. Hope this helps!