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`.
await act(() => {
      updateTransitionPri();
    });
    assertLog([
      
      'Suspend! [Async]',
      'Normal pri: 0',
      'Loading...',
    ]);
    expect(root).toMatchRenderedOutput('(empty), Normal pri: 0');
    await act(async () => {
      await resolveText('Async');
      updateNormalPri();
    });
    assertLog([
      
      '(empty)',
      'Normal pri: 1',
      'Commit',
      
      'Async',
      'Normal pri: 1',
      'Commit',
    ]);
    expect(root).toMatchRenderedOutput('Async, Normal pri: 1');
  });
  it('should not interrupt transitions with normal pri updates', async () => {
    let updateNormalPri;
    let updateTransitionPri;
    function App() {