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`.
'C label',
        'Suspend! [C content]',
        'Loading...',
      ]);
      expect(root).toMatchRenderedOutput(
        <>
          C label (loading...)<div>A content</div>
        </>,
      );
      
      await act(() => {
        resolveText('C content');
      });
      assertLog(['C label', 'C content']);
      expect(root).toMatchRenderedOutput(
        <>
          C label<div>C content</div>
        </>,
      );
    },
  );
  
  
  test(
    'when multiple transitions update the same queue, only the most recent ' +
      'one is allowed to finish (no intermediate states) (classes)',
    async () => {
      let update;
      class App extends React.Component {