Add `scryRenderedComponentsWithRef` and `findRenderedComponentWithRef`
to Test Utils.

This allows for testing that a component has rendered a `ref` without
needing to worry about its contents. E.g., when a child element does not
deserve its own component but yet needs to be checked for existence. This
helper is especially powerful because it is markup agnostic in a similar
fashion to `scryRenderedComponentsWithType` or
`findRenderedComponentWithType`.