Add missing tests for Animated.forkEvent (#20111)
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

exponentjs/react-native,myntra/react-native,arthuralee/react-native,myntra/react-native,janicduplessis/react-native,exponentjs/react-native,myntra/react-native,exponent/react-native,exponent/react-native,myntra/react-native,janicduplessis/react-native,janicduplessis/react-native,pandiaraj44/react-native,exponent/react-native,arthuralee/react-native,myntra/react-native,hammerandchisel/react-native,myntra/react-native,pandiaraj44/react-native,hoangpham95/react-native,hoangpham95/react-native,hoangpham95/react-native,javache/react-native,pandiaraj44/react-native,hammerandchisel/react-native,myntra/react-native,hammerandchisel/react-native,hoangpham95/react-native,javache/react-native,exponentjs/react-native,hammerandchisel/react-native,janicduplessis/react-native,javache/react-native,facebook/react-native,exponent/react-native,hammerandchisel/react-native,pandiaraj44/react-native,hammerandchisel/react-native,hoangpham95/react-native,pandiaraj44/react-native,javache/react-native,arthuralee/react-native,exponentjs/react-native,exponent/react-native,exponentjs/react-native,facebook/react-native,facebook/react-native,hoangpham95/react-native,myntra/react-native,hoangpham95/react-native,janicduplessis/react-native,janicduplessis/react-native,facebook/react-native,hammerandchisel/react-native,facebook/react-native,javache/react-native,exponent/react-native,arthuralee/react-native,exponent/react-native,exponentjs/react-native,hoangpham95/react-native,myntra/react-native,pandiaraj44/react-native,javache/react-native,janicduplessis/react-native,javache/react-native,facebook/react-native,hammerandchisel/react-native,pandiaraj44/react-native,facebook/react-native,javache/react-native,janicduplessis/react-native,exponentjs/react-native,javache/react-native,arthuralee/react-native,facebook/react-native,exponent/react-native,pandiaraj44/react-native,facebook/react-native,exponentjs/react-native