Move out "genMockFunction" and "genMockFn" to "fn"

Summary: In the upcoming Jest version `genMockFunction` and `genMockFn` are deprecated, so we need to kill them.

Reviewed By: rafeca

Differential Revision: D8107155

fbshipit-source-id: 4f46ab58e6e34224eb95e9355385da44f005ea94

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

@@ -81,7 +81,7 @@ describe('Native Animated', () => {
       // via component refs table that we override here.
       c.refs = {
         node: {
-          setNativeProps: jest.genMockFunction(),
+          setNativeProps: jest.fn(),
         },
       };
 

local-cli/util/__mocks__/log.js

@@ -9,5 +9,5 @@
 
 'use strict';
 
-module.exports.out = () => jest.genMockFn();
-module.exports.err = () => jest.genMockFn();
+module.exports.out = () => jest.fn();
+module.exports.err = () => jest.fn();