https://codesandbox.io/s/upbeat-frost-3tt25

1. Press Suspend, see "Loading..."
2. Wait five seconds, you'll see "Happy birthday" at some point 
3. You won't see it again no matter how long you wait

Expected: you'll see it every five seconds.
expect(Scheduler).toFlushAndYield(['A', 'B']);
    expect(ReactNoop.getChildren()).toEqual([span('A'), span('B')]);
    
    ReactNoop.render(<Foo showC={true} />);
    expect(Scheduler).toFlushAndYield([
      'Foo',
      'A',
      'Suspend! [C]',
      'Updating...',
      'B',
    ]);
    
    Scheduler.unstable_advanceTime(600);
    await advanceTimers(600);
    
    
    expect(ReactNoop.getChildren()).toEqual([
      hiddenSpan('A'),
      span('Updating...'),
      span('B'),
    ]);
    
    Scheduler.unstable_advanceTime(5000);
    await advanceTimers(5000);
    expect(Scheduler).toHaveYielded(['Promise resolved [C]']);
    expect(Scheduler).toFlushAndYield(['A', 'C']);
    expect(ReactNoop.getChildren()).toEqual([span('A'), span('C'), span('B')]);
  });
  it('favors showing the inner fallback for nested top level avoided fallback', async () => {
    function Foo({showB}) {