Laravelのfilterはtesting環境では無効になっているので、TestCase内からcsrfフィルターなどのテストをするときはRoute::enableFilters()を呼び出す必要がある。
protected function registerRouter()
{
$this->app['router'] = $this->app->share(function($app)
{
$router = new Router($app['events'], $app);
// If the current application environment is "testing", we will disable the
// routing filters, since they can be tested independently of the routes
// and just get in the way of our typical controller testing concerns.
if ($app['env'] == 'testing')
{
$router->disableFilters();
}