Add frame-driven waitFor utility

[greatestape]

This introduces a waitFor function similar to the one in the react testing library.

I think it's convenient because it should guarantee that we don't miss frames, and it might make tests faster.

Arguably addresses #22

Usage:

  const {lastFrame, waitFor} = render(<MyComponent />);
  await waitFor(() => {
    if (lastFrame() !== 'expected output') {
      throw new Error('not yet');
    }
  });

One big caveat: as you can see in the change, waitFor isn't compatible with AVA tests. As I understand it this is because AVA stores test failures and reports them later, so the "keep trying until assertions pass or timeout" pattern doesn't work.

[sindresorhus]

A few things:

1. This still doesn’t solve the coordination problem it’s trying to solve for input-driven tests. I manually removed the extra tick before stdin.write(...) and the test still failed with Expected "Hello World", got "". So users still need an extra tick just to avoid missing the event subscription.

2. The timeout behavior, docs, and tests need tightening. The implementation always rejects with the last assertion error, not a generic timeout error, so the generic timeout fallback looks unreachable. The current timeout test also only checks that something was thrown, which hides that mismatch.