minor #6381 [Form] [Cookbook] Correctly setup unit tests with dependencies (corphi)

This PR was submitted for the 3.0 branch but it was merged into the 2.8 branch instead (closes #6381).

Discussion
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[Form] [Cookbook] Correctly setup unit tests with dependencies

Among other things, `parent::setUp()` will call `getExtensions()` that uses the private variables.

Commits
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fba2da2 Calling the parent implementation is mandatory.

Author: xabbuh

cookbook/form/unit_testing.rst

@@ -133,6 +133,8 @@ make sure the ``FormRegistry`` uses the created instance::
         {
             // mock any dependencies
             $this->entityManager = $this->getMock('Doctrine\Common\Persistence\ObjectManager');
+
+            parent::setUp();
         }
 
         protected function getExtensions()