Master task: Add tests for every ViewManager, and test setting every prop/event

[kmelmon]

This is a master task to track adding test coverage of ViewManagers that are part of core.

Can leverage some RNTester pages. Will need to create more RNTester pages as well.

Will likely need more infrastructural support as we discover requirements (eg controlling animations, enhancing tree dumps, etc).

Native ViewManagers:

• ActivityIndicator
• Image
• Flyout
• Popup
• Text
• TextInput
• RefreshControl
• ScrollView
• Slider
• Switch
• View

[chrisglein]

Love this. Seems like first step is adding a list of all ViewManagers to this issue (probably as a task list aka checkboxes). Then we can start drilling through them. Of note, we punted on creating a comprehensive list of all apps from a support perspective for API documentation, as it was hard to fish them all out. So we may fall short of a complete list here for similar reasons. But let's start with the list of view managers.

[kmelmon]

@chrisglein What remains here is a strawman schedule and tentative assignments of ViewManagers to devs. Over the next couple weeks I can come up with an estimated schedule.