Fixed falling unit tests on php 5.6

Tests/Twig/Extension/SonataAdminExtensionTest.php

@@ -16,6 +16,7 @@ use Sonata\AdminBundle\Admin\AdminInterface;
 use Sonata\AdminBundle\Admin\FieldDescriptionInterface;
 use Sonata\AdminBundle\Admin\Pool;
 use Sonata\AdminBundle\Exception\NoValueException;
+use Sonata\AdminBundle\Tests\Fixtures\Entity\FooToString;
 use Sonata\AdminBundle\Twig\Extension\SonataAdminExtension;
 use Symfony\Bridge\Twig\Extension\RoutingExtension;
 use Symfony\Bridge\Twig\Extension\TranslationExtension;
@@ -700,12 +701,8 @@ class SonataAdminExtensionTest extends \PHPUnit_Framework_TestCase
                 }
             }));
 
-        $element = $this->getMock('stdClass', array('__toString'));
-        $element->expects($this->any())
-            ->method('__toString')
-            ->will($this->returnValue('bar'));
-
-        $this->assertSame('bar', $this->twigExtension->renderRelationElement($element, $this->fieldDescription));
+        $element = new FooToString();
+        $this->assertSame('salut', $this->twigExtension->renderRelationElement($element, $this->fieldDescription));
     }
 
     public function testRenderRelationElementCustomToString()