Add missing tests for Animated.forkEvent (#20111)

Summary:
`forkEvent` is generally used to intercept an existing listener and add a new js listener to it, considering the original listener can be null/js/Animated.event(). I added tests to ensure the 3 cases of the implementation are all covered.
Pull Request resolved: https://github.com/facebook/react-native/pull/20111

Differential Revision: D8817500

Pulled By: hramos

fbshipit-source-id: 1a20b6f73e2d47bbefccd31378764909a45e89bb

Libraries/Animated/src/__tests__/Animated-test.js

@@ -572,7 +572,7 @@ describe('Animated tests', () => {
       expect(listener.mock.calls.length).toBe(1);
       expect(listener).toBeCalledWith({foo: 42});
     });
-    it('should call forked event listeners', () => {
+    it('should call forked event listeners, with Animated.event() listener', () => {
       const value = new Animated.Value(0);
       const listener = jest.fn();
       const handler = Animated.event([{foo: value}], {listener});
@@ -585,6 +585,24 @@ describe('Animated tests', () => {
       expect(listener2.mock.calls.length).toBe(1);
       expect(listener2).toBeCalledWith({foo: 42});
     });
+    it('should call forked event listeners, with js listener', () => {
+      const listener = jest.fn();
+      const listener2 = jest.fn();
+      const forkedHandler = Animated.forkEvent(listener, listener2);
+      forkedHandler({foo: 42});
+      expect(listener.mock.calls.length).toBe(1);
+      expect(listener).toBeCalledWith({foo: 42});
+      expect(listener2.mock.calls.length).toBe(1);
+      expect(listener2).toBeCalledWith({foo: 42});
+    });
+    it('should call forked event listeners, with undefined listener', () => {
+      const listener = undefined;
+      const listener2 = jest.fn();
+      const forkedHandler = Animated.forkEvent(listener, listener2);
+      forkedHandler({foo: 42});
+      expect(listener2.mock.calls.length).toBe(1);
+      expect(listener2).toBeCalledWith({foo: 42});
+    });
   });
 
   describe('Animated Interactions', () => {