Include tests for Counter as example

[nickserv]

Inspired by reduxjs/cra-template-redux-typescript#21

I'm starting out with integration tests using React Testing Library and Jest (without mocking the store), but I'm open to suggestions. It seems like the existing Redux examples are doing shallow rendering (not using the store) with Enzyme and direct Redux calls for unit tests, but I don't think we should suggest Enzyme or unit testing components. I might add isolated unit tests for the store code if you'd think it's useful though.