Improves testing UIViewController examples

Expands on the work of #151. Currently, we just access the view property and rely on it causing side-effects like `viewDidLoad` to be called, but that's not necessarily the best approach. We're doing something similar on [eidolon](https://github.com/artsy/eidolon/blob/9ee7da7134667e649e8be631e1beface5dde6e8f/KioskTests/ListingsViewControllerTests.swift#L46-L51) and we use the `beginAppearanceTransition` and `endAppearanceTransition` methods. 

These are nice since they are explicit in what they do to the view controller and how they alter its state. Additionally, they call the `viewWillAppear:` and `viewDidAppear:` callbacks, which often have important code to set up the view controller (such as beginner network requests). 

Happy to hear feedback on this!