How to write specs for Symfony2 bundle extension.

AcmeDemoExtension.php

<?php

namespace Acme\DemoBundle\DependencyInjection;

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Loader\XmlFileLoader;
use Symfony\Component\HttpKernel\DependencyInjection\Extension;
use Symfony\Component\Config\FileLocator;

class AcmeDemoExtension extends Extension
{
    public function load(array $configs, ContainerBuilder $container)
    {
        $container->setParameter('acme.test.parameter', 'test');

        $loader = new XmlFileLoader($container, new FileLocator(__DIR__.'/../Resources/config'));
        $loader->load('services.xml');
    }

    public function getAlias()
    {
        return 'acme_demo';
    }
}

AcmeDemoExtensionSpec.php

<?php

namespace spec\Acme\DemoBundle\DependencyInjection;

use PhpSpec\ObjectBehavior;
use Prophecy\Argument;

class AcmeDemoExtensionSpec extends ObjectBehavior
{
    function it_is_initializable()
    {
        $this->shouldHaveType('Acme\DemoBundle\DependencyInjection\AcmeDemoExtension');
    }

    function it_should_set_acme_test_parameter_at_container_builder($builder)
    {
        $builder->beADoubleOf('Symfony\Component\DependencyInjection\ContainerBuilder');
        $builder->hasExtension(Argument::type('string'))->willReturn(false);
        $builder->addResource(Argument::type('\Symfony\Component\Config\Resource\FileResource'))->shouldBeCalled();
        $builder->setDefinition(Argument::type('string'), Argument::type('Symfony\Component\DependencyInjection\Definition'))->shouldBeCalled();

        $builder->setParameter('acme.test.parameter', 'test')->shouldBeCalled()->willReturn($builder);

        $this->load(array(), $builder);
    }

    function it_should_have_acme_demo_alias()
    {
        $this->getAlias()->shouldReturn('acme_demo');
    }
}