The big difference in watchOS 2 is that the code in the watchOS Extension now runs on the actual watch, as opposite as running on the iPhone like it used to. That means a new CPU with a different architecture.
It would be nice testing that code on an actual device before shipping it, right?

Well, we are out of luck since it seems XCTest does not support watchOS yet. There’s no XCTest framework in the watchOS SDK and trying to use it causes compiler errors, since the headers only support either UIKit or AppKit (and watchOS has none).

I duped rdar://21760513. You should probably do that too!