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
a455520ecb
