Add `scryRenderedComponentsWithRef` and `findRenderedComponentWithRef`.

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`.