bug #27352 Remove reference to the test container after kernel shutdown (stof)

This PR was merged into the 4.1 branch.

Discussion
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Remove reference to the test container after kernel shutdown

| Q             | A
| ------------- | ---
| Branch?       | 4.1
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

This reference is not working anymore, as the main container was reset and cleaned. Keeping a reference to the test container will prevent collecting the object graph.

Commits
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d8fb1b97c9 Remove reference to the test container after kernel shutdown

src/Symfony/Bundle/FrameworkBundle/Test/KernelTestCase.php

@@ -123,6 +123,7 @@ abstract class KernelTestCase extends TestCase
                 $container->reset();
             }
         }
+        static::$container = null;
     }
 
     /**