Modernize tests

[rpiaggio]

This PR is a step forward into removing ReactTestUtils in favor of the newer ReactTestUtils2 (which will be renamed once the former is removed).

It also removes all uses of the sync version of act in favor of the async one, given that the React docs specify that:

We recommend using act with await and an async function. Although the sync version works in many cases, it doesn’t work in all cases and due to the way React schedules updates internally, it’s difficult to predict when you can use the sync version.

We will deprecate and remove the sync version in the future.

A number of scaffolding code was also implemented:

• AsyncTestSuite allows running tests that return an AsyncCallback and runs them. This is not published but rather copied and pasted whenever it's used. I'm not sure where to place it and it would probably need to be in its own scalajs-react-utest-acb bundle.
• polyfill.js now provides access to MessageChannel, which allows executing async/await within jsdom. Again, this is copied in the few test project where it is used.
• Resource implements a managed resource. It is a generic (and therefore async) version of WithDsl. I think WithDsl can be replaced in the future with Resource[Id]. However, we may end up deprecating and removing WithDsl together with the sync version of act. This class only lives in the testing utils project, but it may be useful for other things (like AroundReact). If we decide to adopt it for client use, we may want to polish it up a bit. Eg: build an AST and then interpret it, rather than using vars, and making sure it is stack safe.

The few places where ReactTestUtils is still used is where class components are tested. These tests need access to the mounted versions of the components, which the modern testing framework doesn't seem to provide. Probably the course of action here will be just to remove these tests, or move them all, together with ReactTestUtils to a tests-legacy project. I'm open to suggestions.

[rpiaggio]

Doc update is pending.

[toddburnside]

Wow! That was a lot of work. LGTM.

[toddburnside]

Can this file go away?

[rpiaggio]

We will reinstate it once we release 3.0.0.