Inconsistent behaviour with async tests

[njoyard]

Test scenario:

• Register the ember-raf-scheduler waiter in tests/test-helper.js
• Use the following test:

  test('vertical-collection async test', async function(assert) {
    let items = A()

    function loadMore() {
      items.pushObjects([...Array(10).keys()].map(() => Math.random()))
    }

    loadMore()
    this.setProperties({
      items,
      lastReached() {
        loadMore()
      }
    })

    await render(hbs`
      <div class="vertical-collection">
        {{#vertical-collection
          items
          estimateHeight='100px'
          containerSelector='#ember-testing-container'
          renderAll=true
          lastReached=lastReached
          as |item|}}
          <div style="height: 100px;" class="item">{{item}}</div>
        {{/vertical-collection}}
      </div>
    `)

    assert.dom('.item').exists({ count: 10 })

    document.querySelector('.vertical-collection').scrollIntoView(false)
    await settled()

    assert.dom('.item').exists({ count: 20 })
  })

Running this test repeatedly fails randomly on the second assert (only 10 items present). As I understand it, registering the waiter should be enough that awaiting for settled() (or other ember async helpers) ensures that vertical-collection updates are finished. But maybe I am wrong?

May be related to #231 ?