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 waitFor([
        
        
        'Suspend! [Async]',
        'Loading...',
        'After Suspense',
      ]);
      
      startTransition(async () => {
        root.render(
          <>
            <Suspense fallback={<Text text="Loading..." />} />
            <Text text="Updated" />
          </>,
        );
      });
    });
    
    
    assertLog(['Updated']);
    expect(root).toMatchRenderedOutput('Updated');
  });
  
  test(
    "interrupt a refresh transition when something suspends and we've " +
      'already bailed out on another transition in a parent',
    async () => {
      let setShouldSuspend;
      function Parent({children}) {