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`.
</Suspense>
            </div>
          </>
        );
      }
      
      const root = ReactNoop.createRoot();
      await act(() => {
        seedNextTextCache('A content');
        root.render(<App />);
      });
      assertLog(['A label', 'A content']);
      expect(root).toMatchRenderedOutput(
        <>
          A label<div>A content</div>
        </>,
      );
      
      await act(() => {
        update('B');
      });
      assertLog([
        
        'B label (loading...)',
        'A content',
        
        'B label',
        'Suspend! [B content]',
        'Loading...',
      ]);