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(() => {
        root.render(<App />);
      });
      assertLog([]);
      expect(root).toMatchRenderedOutput(null);
      
      await act(() => {
        startTransition(() => {
          setShowA(true);
        });
      });
      assertLog(['Suspend! [A]', 'Loading...']);
      expect(root).toMatchRenderedOutput(null);
      
      await act(() => {
        startTransition(() => {
          setShowA(false);
          setShowB(true);
        });
      });
      assertLog(['Suspend! [B]', 'Loading...']);
      expect(root).toMatchRenderedOutput(null);
      
      
      await act(() => {
        startTransition(() => {
          setShowB(false);