Fix integration test

The Mock IdP must set a Destination attribute in the <Response>, as per simplesamlphp/saml2@b4daf21

src/OpenConext/EngineBlockFunctionalTestingBundle/Controllers/IdentityProviderController.php

@@ -99,6 +99,10 @@ public function singleSignOnAction(Request $request, $idpName)
         $destination = ($authnRequest->getAssertionConsumerServiceURL() ?
                 $authnRequest->getAssertionConsumerServiceURL() :
                 $response->getDestination());
+            $response->getDestination());
+
+        /* set the destination element of the response to the ACS URL */
+        $response->setDestination($destination);
 
         if ($mockIdp->mustUseHttpRedirect()) {
             $redirect = new HTTPRedirect();