How can i override user attribute in my functional test?

class myBrowser extends sfBrowser
{
  /**
   * Calls a request to a uri.
   */
  protected function doCall()
  {
    // manually shutdown user to save current session data
    if (isset($this->context) AND $this->context->getUser())
    {
      $this->context->getUser()->shutdown();
      $this->context->getStorage()->shutdown();
    }
 
    // recycle our context object
    $this->context = $this->getContext(true);
 
    sfConfig::set('sf_test', true);
 
    // we register a fake rendering filter
    sfConfig::set('sf_rendering_filter', array('sfFakeRenderingFilter', null));
 
    $this->resetCurrentException();
 
    // dispatch our request
    ob_start();
    $this->context->getController()->dispatch();
    $retval = ob_get_clean();
 
    // append retval to the response content
    $this->context->getResponse()->setContent($retval);
  }
}

include dirname(__FILE__).'/../../bootstrap/functional.php';
 
$browser = new sfTestFunctional(new myBrowser());
 
$browser->
  get('/site-one');
 
// override the session data
$browser->getUser()->setAttribute('key', $value);
 
$browser->
  get('/site-one');