title: Best way to find a non-stubbed transition wrapper with vue-test-utils v1
body: <p>I am upgrading a very large application from vue-test-utils beta to v1. Previously when testing transitions vue-test-utils had TransitionStub and TransitionGroupStub components pre-stubbed and available to find and interact with transitions. I know I can create my own... </p>
  
  <pre><code>const TransitionStub = {
      render() {
          return this.$options._renderChildren;
      },
  };
  </code></pre>
  
  <p>which works for simple situations, but if I shallowMount a component with a transition and want to emit an <code>after-enter</code> event to test what occurs after, what would make the most sense? This component currently renders the transition like so when not stubbed:</p>
  
  <p><code>&lt;div name="opacity" enter-class="opacity-enter" enter-active-class="opacity-enter-active" leave-active-class="opacity-leave-active" leave-to-class="opacity-leave-to" mode="out-in"&gt;</code></p>
  
  <p>and I can do <code>wrapper.findAll('div')</code> and emit the <code>after-enter</code> event where I would previously do <code>wrapper.findAll(TransitionStub)</code>, but finding a transition by it's tag is very much so not a trustworthy strategy. </p>
  
  <p>If I stub the transition with the above code, I'm getting a <code>[Vue warn]: Multiple root nodes returned from render function. Render function should return a single root node.</code> in my test. Another complication here is that the transitions are inside table columns, though I am only passing data for one column in this test. </p>