Update documentation for testing dependency with Enzyme

[josephrace]

Can you reproduce the problem with latest npm?

Yes

Description

With React 15.5, React Test Utils have moved to react-dom/test-utils, the react-addons-test-utils package is deprecated and the shallow renderer has been moved to react-test-renderer/shallow (https://facebook.github.io/react/blog/2017/04/07/react-v15.5.0.html).

The information in the User Guide should be updated to instruct users to install react-test-renderer instead of react-addons-test-utils.

Expected behavior

With freshly bootstrapped app and with enzyme and react-addons-test-utils installed as instructed, the following test should pass:

import React from 'react';
import { shallow } from 'enzyme';
import App from './App';

it('renders without crashing', () => {
  shallow(<App />);
});

Actual behavior

I see the following warnings in the console:

console.error node_modules/fbjs/lib/warning.js:36
    Warning: ReactTestUtils has been moved to react-dom/test-utils. Update references to remove this warning.

console.error node_modules/fbjs/lib/warning.js:36
    Warning: Shallow renderer has been moved to react-test-renderer/shallow. Update references to remove this warning.

If I uninstall react-addons-test-utils then Enzyme shows this warning:

 console.error node_modules/enzyme/build/react-compat.js:148
    react-dom@15.5+ and react-test-renderer are implicit dependencies when usingreact@15.5+ with enzyme. Please add the appropriate version to yourdevDependencies. See https://github.com/airbnb/enzyme#installation

If I install react-test-renderer then test passes:

 PASS  src/App.test.js
  ✓ renders without crashing (8ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        2.351s
Ran all test suites.

Environment

1. npm ls react-scripts (if you haven’t ejected): react-scripts@0.9.5
2. node -v: v6.9.1
3. npm -v: 4.5.0