Native Animated - Add tests on iOS

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

Libraries/NativeAnimation/RCTNativeAnimatedModule.m

@@ -161,9 +161,8 @@ RCT_EXPORT_METHOD(startListeningToAnimatedNodeValue:(nonnull NSNumber *)tag)
 
 RCT_EXPORT_METHOD(stopListeningToAnimatedNodeValue:(nonnull NSNumber *)tag)
 {
-  __weak id<RCTValueAnimatedNodeObserver> valueObserver = self;
   [self addOperationBlock:^(RCTNativeAnimatedNodesManager *nodesManager) {
-    [nodesManager stopListeningToAnimatedNodeValue:tag valueObserver:valueObserver];
+    [nodesManager stopListeningToAnimatedNodeValue:tag];
   }];
 }