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`.
assertLog(['C']);
      expect(root).toMatchRenderedOutput('C');
    },
  );
  
  test('interrupt a refresh transition if a new transition is scheduled', async () => {
    const root = ReactNoop.createRoot();
    await act(() => {
      root.render(
        <>
          <Suspense fallback={<Text text="Loading..." />} />
          <Text text="Initial" />
        </>,
      );
    });
    assertLog(['Initial']);
    expect(root).toMatchRenderedOutput('Initial');
    await act(async () => {
      
      startTransition(() => {
        root.render(
          <>
            <Suspense fallback={<Text text="Loading..." />}>
              <AsyncText text="Async" />
            </Suspense>
            <Text text="After Suspense" />
            <Text text="Sibling" />
          </>,
        );
      });