Summary:
Adds unit tests to the Native Animated implementation on iOS. This pretty much mirrors the tests we currently have on Android.
It also fixes 2 bugs I've found when adding the tests and pass the current time in `stepAnimation` instead of using `CACurrentMediaTime` to make testing easier.
- `stopListeningToAnimatedNodeValue` did not actually work at all, it should set the listener to nil.
- The finished value in the animation end callback was always true, this simplifies the `RCTAnimationDriver` interface to get rid of `removeAnimation` and fixes the end callback value.
**Test plan**
- Run the tests
- Make sure the UIExplorer example still works
Closes https://github.com/facebook/react-native/pull/13068
Differential Revision: D4786701
Pulled By: javache
fbshipit-source-id: a4f07e6eec1f363ca47b6f27984041793c915bfc
1d37dd063c