fix mocks

* check for existance of `setMetadataFactory()` method (this is needed
  for tests run with deps=high as the method was removed in Symfony
  3.0)
* fix mock testing the `EngineInterface` as the `stream()` method cannot
  be mocked when it is does not exist in the mocked interface

Author: xabbuh

Tests/Extension/Validator/ValidatorExtensionTest.php

@@ -12,6 +12,7 @@
 namespace Symfony\Component\Form\Tests\Extension\Validator;
 
 use Symfony\Component\Form\Extension\Validator\ValidatorExtension;
+use Symfony\Component\Validator\ValidatorInterface;
 
 class ValidatorExtensionTest extends \PHPUnit_Framework_TestCase
 {
@@ -38,9 +39,11 @@ public function test2Dot5ValidationApi()
             ->method('addPropertyConstraint')
             ->with('children', $this->isInstanceOf('Symfony\Component\Validator\Constraints\Valid'));
 
-        $validator
-            ->expects($this->never())
-            ->method('getMetadataFactory');
+        if ($validator instanceof ValidatorInterface) {
+            $validator
+                ->expects($this->never())
+                ->method('getMetadataFactory');
+        }
 
         $extension = new ValidatorExtension($validator);
         $guesser = $extension->loadTypeGuesser();