minor #6391 Fix mem leak in example doctrine testing (nicolas-grekas)

This PR was merged into the 2.3 branch.

Discussion
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Fix mem leak in example doctrine testing

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | symfony/symfony#18236

Commits
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7be63a4 Fix mem leak in example doctrine testing

Author: wouterj

cookbook/testing/doctrine.rst

@@ -62,5 +62,6 @@ which makes all of this quite easy::
         {
             parent::tearDown();
             $this->em->close();
+            $this->em = null;
         }
     }