Add find/scry by ID to test utils

[feifanzhou]

TestUtils currently has scryRenderedDOMComponentsWithClass and the find variant, but nothing like findRenderedDOMComponentWithID. This functionality would make testing a lot easier.

[sophiebits]

We're not planning to add more utils like this to TestUtils. You can always implement this locally in your project as a helper if you want to, but we're planning to move towards shallow testing (i.e., TestUtils. createRenderer) in the future.