Shallow renderer and test utils bundles
Adds new bundles introduced with React 15.5 release to master (and 16 alpha)
react-dom/test-utils:
This new bundle contains what used to be react-addons-test-utils. This bundle shares things from react-dom rather than duplicates them.
A temporary createRenderer method has been left behind as a way to access the new shallow renderer. This is for the ReactNative release cycle only and should be going away before the final release.
react-test-renderer/shallow:
This new shallow renderer is almost entirely stand-alone (in that it doesn't use the React reconciler or scheduler). The only touch points are ReactElement and prop/context validation. This renderer is stack and fiber compatible.
