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(async () => {
        root.render(<App />);
        await waitForAll(['A', 'shouldHideInParent: false', 'B', 'C']);
        expect(root).toMatchRenderedOutput('ABC');
        
        startTransition(() => {
          setShouldSuspend(true);
        });
        
        
        
        
        await waitFor(['A']);
        
        React.startTransition(() => {
          setShouldHideInParent(true);
        });
        
        await waitFor([
          'shouldHideInParent: false',
          'Suspend! [Async]',
          'Loading...',
          'B',
        ]);
        
        expect(root).toMatchRenderedOutput('ABC');