Update to Jest 27

Created by: gaearon

There's a bugfix in React that we need to make. Unfortunately, there's a risk of that change breaking some tests. In order to fix the tests, you'd have to upgrade Jest. But if you use Jest via CRA, you can't do this directly.

We need to upgrade CRA to Jest 27 so that we can unblock React from making this bugfix. Jest 27 isn't out yet, but we can start testing the integration using the 27.0.0-next.6 version.