title: Test TwigExtension with assetHelper in Symfony2
body: <p>I have a Twig extension and a couple of functions what retruns something like, </p>
  
  <p><code>&lt;script type="text/javascript" src="'.$this-&gt;assetHelper-&gt;getAssetUrl('bundles/mybundle/js/script.js').'"&gt;&lt;/script&gt;</code></p>
  
  <p>But assetHelper require request service. Tests I run in cli, so there is no Request object. How I can test this function? </p>