adding jest test setup

Summary:
Adds a test runner for jest test and adds three simple test cases:
- render the app
- start a server
- client connecting to the app

Test can be run using `yarn test`.

To make the test runner work, some changes needed to be made:
- remove the export of `init()` from `'flipper'`, because it was a cyclic dependency
- updating Button.js to the new ref-API

Reviewed By: jknoxville

Differential Revision: D10027078

fbshipit-source-id: 49107b0dd4dec666b92ecd841422fe7e6b3a7756

src/init.js

@@ -62,7 +62,7 @@ const AppFrame = () => (
   </TooltipProvider>
 );
 
-export default function init() {
+function init() {
   // $FlowFixMe: this element exists!
   ReactDOM.render(<AppFrame />, document.getElementById('root'));
   // $FlowFixMe: service workers exist!
@@ -77,3 +77,6 @@ export default function init() {
     })
     .catch(console.error);
 }
+
+// make init function callable from outside
+window.Flipper.init = init;