print branch name for testing

[kaylangan]

Before submitting a pull request, please make sure the following is done:

1. Fork the repository and create your branch from master.
2. Run yarn in the repository root.
3. If you've fixed a bug or added code that should be tested, add tests!
4. Ensure the test suite passes (yarn test). Tip: yarn test --watch TestName is helpful in development.
5. Run yarn test-prod to test in the production environment. It supports the same options as yarn test.
6. If you need a debugger, run yarn debug-test --watch TestName, open chrome://inspect, and press "Inspect".
7. Format your code with prettier (yarn prettier).
8. Make sure your code lints (yarn lint). Tip: yarn linc to only check changed files.
9. Run the Flow typechecks (yarn flow).
10. If you haven't already, complete the CLA.

Learn more about contributing: https://reactjs.org/docs/how-to-contribute.html