Improve async component mounting

[SHA-4]

This PR fixes some flakiness we had with some of our tests, mainly by addressing enzymejs/enzyme#1587 and by awaiting the componentDidMount of our component.

It also makes tester mount async by default, simplifying our tests.

Example code before:

const tester = await new Tester(MyComponent).mount({ async: true });
await tester.refresh(100); // this would be a guess for how long we needed to wait

Example code after:

const tester = await new Tester(MyComponent).mount();