I got an error:
Undefined symbols for architecture x86_64: \"DirectBistro.DBTabBarOrderedIndexesKey.unsafeMutableAddressor : Swift.String\", ref
The conslusion is: it is not going to work.
I report it as a bug to Apple, and got a response:
UI tests execute differently from Unit tests - Unit tests run inside your application process so they can access your application code. UI tests execute in a separate process, outside your application, so they can simulate how the user interacts with the application. It’s not expected that you will be able to access your app class from a UI test.