Add tests to create-edx-react-app

[adamstankiewicz]

Right now, create-edx-react-app does not have any tests. Let's add some! 🎉 😉

[jaebradley]

I think it'd be useful to itemize the different tests / groups of tests you'd want or expect to be a part of the same PR so that it's easy to see what has been done / needs to be done.