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/Drivers/RCTSpringAnimation.m

@@ -71,19 +71,12 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
 
 - (void)startAnimation
 {
-  _animationStartTime = CACurrentMediaTime();
-  _animationCurrentTime = _animationStartTime;
+  _animationStartTime = _animationCurrentTime = -1;
   _animationHasBegun = YES;
 }
 
 - (void)stopAnimation
 {
-  _animationHasFinished = YES;
-}
-
-- (void)removeAnimation
-{
-  [self stopAnimation];
   _valueNode = nil;
   if (_callback) {
     _callback(@[@{
@@ -92,13 +85,17 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
   }
 }
 
-- (void)stepAnimation
+- (void)stepAnimationWithTime:(NSTimeInterval)currentTime
 {
   if (!_animationHasBegun || _animationHasFinished) {
     // Animation has not begun or animation has already finished.
     return;
   }
 
+  if (_animationStartTime == -1) {
+    _animationStartTime = _animationCurrentTime = currentTime;
+  }
+
   // We are using a fixed time step and a maximum number of iterations.
   // The following post provides a lot of thoughts into how to build this
   // loop: http://gafferongames.com/game-physics/fix-your-timestep/
@@ -106,7 +103,6 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
   // Velocity is based on seconds instead of milliseconds
   CGFloat step = TIMESTEP_MSEC / 1000;
 
-  NSTimeInterval currentTime = CACurrentMediaTime();
   NSInteger numSteps = floorf((currentTime - _animationCurrentTime) / step);
   _animationCurrentTime = currentTime;
   if (numSteps == 0) {
@@ -182,7 +178,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
       [self onUpdate:_toValue];
     }
 
-    [self stopAnimation];
+    _animationHasFinished = YES;
   }
 }