title: How to change controller's property value inside acceptance test?
body: <p>Is there a way to alter the controller's property value inside acceptance test?</p>
  
  <pre><code>test('should add new post', function(assert) {
    visit('/posts/new');
    fillIn('input.title', 'My new post');
    click('button.submit');
    andThen(() =&gt; assert.equal(find('ul.posts li:first').text(), 'My new post'));
  });
  </code></pre>
  
  <p>For example, I would like to set the default value for an input before running the test.</p>