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`.
Scheduler.log(text);
    return text;
  }
  function AsyncText({text}) {
    readText(text);
    Scheduler.log(text);
    return text;
  }
  function seedNextTextCache(text) {
    if (seededCache === null) {
      seededCache = createTextCache();
    }
    seededCache.resolve(text);
  }
  function resolveText(text) {
    if (caches.length === 0) {
      throw Error('Cache does not exist.');
    } else {
      
      
      caches[caches.length - 1].resolve(text);
    }
  }
  
  test('isPending works even if called from outside an input event', async () => {
    let start;
    function App() {