Provide a full end to end example

I'm a long-time Bazel user getting into iOS development. Some of the terms are still mysterious to me. I think it would be helpful to this project and to bazelbuild/rules_apple to provide an end to end example of how to use xctestrunner.

* What is a app under test?
* What's required? How much needs to be stubbed out?
* Provide a minimal template that can be used directly or extended?
* What is the test bundle?
* How do they fit together with xctestrunner to run on device.
* Is it possible to build a multi-arch binary so that the same test can be run on both simulator and device?