Add Jest.

Created by: cpojer

This adds Jest as a default test runner with an example snapshot test as announced in the Jest 14 blog post. The generated snapshot for the example looks like this:

exports[`App renders a welcome view 1`] = `
<div
  className="App">
  <div
    className="App-header">
    <img
      alt="logo"
      className="App-logo"
      src="test-file-stub" />
    <h2>
      Welcome to React
    </h2>
  </div>
  <p
    className="App-intro">
    To get started, edit 
    <code>
      src/App.js
    </code>
     and save to reload.
  </p>
</div>
`;

This integration is the simplest test runner integration I can think of and requires no configuration in the project itself. The repo internal configuration mirrors some of the configuration from the webpack integration tutorial.

I understand there are many opinions in this space and many people have aspirational goals as to what a test runner and framework should be. In reality, what we have found at Facebook is that when it is hard to set up a test runner or hard to write tests, people simply don't do it. This is why Jest's philosophy is to provide an integrated experience that is well set-up and performant by default.

Further, I believe that snapshot testing and the React test renderer will greatly influence how people write React tests and I'm excited for the possibility of other test runners integrating better with this in the future. DOM testing should be left to webdriver or other end-to-end tests. In a test environment, regardless of the real rendering target, there is little value in rendering to a fake DOM. This is especially true for non-web rendering targets.