Created by: robbertvanginkel
This allows apple_test bundles with is_ui_test to be correctly build
with buck, so that they can be used with an external test runner.
Currently, the test_host_app is used as the bundle_loader when
linking an apple_test. Since the uitest bundle isn't injected into the
app it is testing, but into a stub app when building a uitest we should
not use it as the bundle_loader.