bug symfony#60693 [FrameworkBundle] ensureKernelShutdown in tearDownAfterClass (cquintana92)

This PR was submitted for the 7.4 branch but it was squashed and merged into the 6.4 branch instead.

Discussion
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[FrameworkBundle] ensureKernelShutdown in tearDownAfterClass

| Q             | A
| ------------- | ---
| Branch?       | 6.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Issues        | None
| License       | MIT

Comes from symfony#60564

This PR performs the following change: In `KernelTestCase`, the kernel is shut down after every test, as per the `static::ensureKernelShutdown()` call in the `tearDown` function. However, if one was to perform any extra code in their `tearDownAfterClass` and call `getContainer`, it would boot up a new kernel, which would then be left dangling. The main issue with that is that when the next Test class is executed, there would already be a booted kernel, so it could happen that it was pointing to dangling resources.

By adding this call to `static::ensureKernelShutdown` in the `tearDownAfterClass`, we can ensure that no dangling kernels are left even after the test class has finished.

Commits
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c193b98 [FrameworkBundle] ensureKernelShutdown in tearDownAfterClass

Author: nicolas-grekas

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

@@ -45,6 +45,14 @@ protected function tearDown(): void
         static::$booted = false;
     }
 
+    public static function tearDownAfterClass(): void
+    {
+        static::ensureKernelShutdown();
+        static::$class = null;
+        static::$kernel = null;
+        static::$booted = false;
+    }
+
     /**
      * @throws \RuntimeException
      * @throws \LogicException