Add parity test suite for ReactDOMServer

[aweary]

It's important that rapscallion produce output consistent with ReactDOMServer.renderToString. We should add a suite of tests that renders a number of different types of components and layouts and verify that the output matches.

[aickin]

FYI, I'm currently porting over the ~550 server rendering tests I wrote for react-dom-stream into react master. A PR with the first 16 tests was accepted into react today, and I'll continue to add them over the next week, hopefully. I though they might help you, and I'd be happy to give you pointers on how to integrate them if you want.

If you're interested in what the tests look like, they are here: https://github.com/aickin/react/blob/server-render-unit-tests-async-await-2/src/renderers/dom/shared/tests/ReactDOMServerIntegration-test.js