WebTestCase.php

<?php

namespace MyNamespace\CoreBundle\Test;

use Symfony\Bundle\FrameworkBundle\Client;
use Symfony\Bundle\FrameworkBundle\Test\WebTestCase as SymfonyWebTestCase;
use Symfony\Component\Process\Process;

abstract class WebTestCase extends SymfonyWebTestCase
{
    protected $container;

    // feel free to adapt this method, main point is that you get container in $this->container
    protected function init()
    {
        $kernel = static::createKernel();
        $kernel->boot();
        $this->container = $kernel->getContainer();
    }

    protected function saveOutput(Client $client, $domain = null, $browser = 'firefox')
    {
        $path = $this->container->get('kernel')->getRootDir() . '/../web/test_out.html';

        file_put_contents($path, $client->getResponse()->getContent());

        if (!is_null($domain)) {
            $process = new Process($browser . ' "'.$domain.'/test_out.html"');
            $process->run();
        }
    }
}

WebTestCase2.php

<?php

namespace MyNamespace\CoreBundle\Test;

use Symfony\Bundle\FrameworkBundle\Client;
use Symfony\Bundle\FrameworkBundle\Test\WebTestCase as SymfonyWebTestCase;
use Symfony\Component\Process\Process;

abstract class WebTestCase extends SymfonyWebTestCase
{
    protected function saveOutput(Client $client, $domain = null, $delete = true, $browser = 'firefox')
    {
        $path = $this->container->get('kernel')->getRootDir() . '/../web/test_out.html';

        file_put_contents($path, $client->getResponse()->getContent());

        if (!is_null($domain)) {
            $process = new Process($browser . ' "'.$domain.'/test_out.html"');
            $process->start();
            sleep(1);

            if ($delete) {
                unlink($path);
            }
        }
    }
}

WebTestCase3.php

<?php

namespace MyNamespace\CoreBundle\Test;

use Symfony\Bundle\FrameworkBundle\Client;
use Symfony\Bundle\FrameworkBundle\Test\WebTestCase as SymfonyWebTestCase;
use Symfony\Component\Process\Process;

abstract class WebTestCase extends SymfonyWebTestCase
{
    protected function saveOutput(Client $client, $domain = null, $delete = true, $browser = 'firefox')
    {
        $path = $this->container->get('kernel')->getRootDir() . '/../web/test_out.html';

        file_put_contents($path, $client->getResponse()->getContent());

        if (!is_null($domain)) {
            $process = new Process($browser . ' "'.$domain.'/test_out.html"');
            $process->run(function() use($path, $delete) {
                if ($delete) {
                    unlink($path);
                }
            });
        }
    }
}