There is no easy way to do what you ask. What is normally done is truncating the database before and after executing your tests so you have a truly clean and isolated environment.
Quoting from this nice article (http://blog.sznapka.pl/fully-isolated-tests-in-symfony2/:
public function setUp()
{
$kernel = new \AppKernel("test", true);
$kernel->boot();
$this->_application = new \Symfony\Bundle\FrameworkBundle\Console\Application($kernel);
$this->_application->setAutoExit(false);
$this->runConsole("doctrine:schema:drop", array("--force" => true));
$this->runConsole("doctrine:schema:create");
$this->runConsole("doctrine:fixtures:load", array("--fixtures" => __DIR__ . "/../DataFixtures"));
}
As you can see, this solution makes use of Doctrine's Symfony2 commands in order to achieve the isolation state. There is a bundle I like to use which solves exactly this problem and let you use nice ready to use FunctionalTest base classes and many other features. Check it out: