Nested `startTransition` call "takes over" and makes parent `startTransition` unable to track `isPending`.

Seb says it's a bug.

Repro:

https://codesandbox.io/s/pensive-breeze-rg70wn?file=/IndexPage.js:192-288

1. Click the button
2. `isPending` in `IndexPage.js` is `true`

However, `isPending` in `App.js` is `false`.

Expected: `isPending` in `App.js` is also `true`.
);
    }
    const root = ReactNoop.createRoot();
    await act(() => {
      root.render(<App />);
    });
    
    assertLog(['Transition pri: 0', 'Normal pri: 0', 'Commit']);
    expect(root).toMatchRenderedOutput('Transition pri: 0, Normal pri: 0');
    await act(() => {
      updateTransitionPri();
      updateNormalPri();
    });
    assertLog([
      
      'Transition pri: 0',
      'Normal pri: 1',
      'Commit',
      
      'Transition pri: 1',
      'Normal pri: 1',
      'Commit',
    ]);
    expect(root).toMatchRenderedOutput('Transition pri: 1, Normal pri: 1');
  });
  
  it('should render normal pri updates before transition suspense retries', async () => {
    let updateTransitionPri;
    let updateNormalPri;
    function App() {