Test isolation with many requests on Symfony 2.6

My project uses Symfony 2.6. I am trying to isolate my test in order to don't commit any change on my database.

I manage to isolate my test with only one request. But when there are many of them, it doesn't work and I can't manage to find why. Any ideas ?

Here is my testController code :

use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;

class RequestControllerTest extends WebTestCase{

    protected $client;
    protected $entityManager;

    protected function setUp(){
        parent::setUp();
        $this->client = static::createClient();
        $this->entityManager = $this->client->getContainer()->get('doctrine')->getManager();
        $this->entityManager->beginTransaction();
    }

    protected function tearDown(){
        parent::tearDown();
        $this->entityManager->rollback();
        $this->entityManager->close();
    }

    // This test is working just fine : the request isn't deleted from database at the end
    public function testIsolationOK(){
       $this->client->request('DELETE', self::WEB_SERVICES_EXISTING_REQUEST_URL);
       $this->client->request('GET', self::WEB_SERVICES_EXISTING_REQUEST_URL);           
    }

    // But this one isn't working : the request is deleted from database at the end
    public function testIsolationNOK(){
       $this->client->request('GET', self::WEB_SERVICES_EXISTING_REQUEST_URL);
       $this->client->request('DELETE', self::WEB_SERVICES_EXISTING_REQUEST_URL);
    }

}