Isolate ReactUpdates-test cases

This ensures their behavior is consistent when run in isolation, and that they actually test the cases they're describing.

Author: gaearon

packages/react-dom/src/__tests__/ReactUpdates-test.js

@@ -15,6 +15,7 @@ let ReactTestUtils;
 
 describe('ReactUpdates', () => {
   beforeEach(() => {
+    jest.resetModules();
     React = require('react');
     ReactDOM = require('react-dom');
     ReactTestUtils = require('react-dom/test-utils');