UI testing for the demo app

[JakeLin]

Following up #450, when we do refactor, we may accidently break some features.

I don't know the best practice for unit testing UI. We developed a demo app (IBAnimatable Playground) to test our features. We use it for testing new features, fixing bugs and doing refactor. But it is not straight forward and we have to check them manually. If we can have some UI Testing in Xcode. We may be able to integrate it with CI and check for every PR.

We don't have to do it all in one go. We can develop it bit by bit:

• User Interface: It has a list of UI components like placeholder, mask, etc, we can do it one by one.
• Animations: Changing the picker view.
• Activity Indicator: Changing the picker view. That would be an easy one to start with
• Refresh Control: Pulling the table view to refresh.
• Transitions: Clicking on the table view cell.
• Presentations
• Interactions

[tbaranes]

Even if the current state of XCUITest are a bit limited (can't test the layer, the animations...), we can still do basic animations with a simple navigation which ensure us to be able to launch each feature without crashing.

Let's hope that will be improved in the future version of XCUITest!