Feature: Add find* to gain alignment with react-test-renderer

It seems that react-test-renderer exports this package under `react-test-renderer/shallow` to support shallow rendering. However this package doesn't contain the low level "find" functions that make it easy to dive into a shallow component similar to `react-test-renderer`'s default export.

The ask is to implement the following functions for consistency:
- [testInstance.find()](https://reactjs.org/docs/test-renderer.html#testinstancefind)
- [testInstance.findByType()](https://reactjs.org/docs/test-renderer.html#testinstancefindbytype)
- [testInstance.findByProps()](https://reactjs.org/docs/test-renderer.html#testinstancefindbyprops)
- [testInstance.findAll()](https://reactjs.org/docs/test-renderer.html#testinstancefindall)
- [testInstance.findAllByType()](https://reactjs.org/docs/test-renderer.html#testinstancefindallbytype)
- [testInstance.findAllByProps()](https://reactjs.org/docs/test-renderer.html#testinstancefindallbyprops)

I know enzyme "provides a nicer higher-level API over the same functionality" my ask is to provide even basic low level to have alignment between shallow and deep core rendering engines.

I think we could probably just copy most of the logic for the deep renderer [here](https://github.com/facebook/react/blob/8e2bde6f2751aa6335f3cef488c05c3ea08e074a/packages/react-test-renderer/src/ReactTestRenderer.js#L355) and it should get us 80% of the way.

Then in the future Enzyme could update to use these lower level functions to help fuel it's higher level functions.