AppKernel.php

<?php

use Symfony\Component\HttpKernel\Kernel;
use Symfony\Component\Config\Loader\LoaderInterface;

class AppKernel extends Kernel
{
    static protected $databaseDefaultConnection;

    public function registerBundles()
    {
        // ...
    }

    public function boot()
    {
        parent::boot();

        if ('test' === $this->getEnvironment()) {
            if (null === static::$databaseDefaultConnection) {
                static::$databaseDefaultConnection = $this->getContainer()->get('doctrine.dbal.default_connection');
            } else {
                $this->getContainer()->set('doctrine.dbal.default_connection', static::$databaseDefaultConnection);
            }
        }
    }

    public function registerContainerConfiguration(LoaderInterface $loader)
    {
        $loader->load(__DIR__.'/config/config_'.$this->getEnvironment().'.yml');
    }
}