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/index.js

@@ -28,5 +28,3 @@ export {default as DetailSidebar} from './chrome/DetailSidebar.js';
 export {default as AndroidDevice} from './devices/AndroidDevice.js';
 export {default as Device} from './devices/BaseDevice.js';
 export {default as IOSDevice} from './devices/IOSDevice.js';
-
-export {default as init} from './init.js';