Symfony version(s) affected

4.4.39

Description

I just updated an ancient Symfony 4.4 app and noticed that in release 4.4.39 out unit tests break due to symfony/framework-bundle@cd52737

After debugging I noticed that this happens because bootKernel assigns the new kernel to self::$kernel, but a few lines down, the container is read from static::$kernel, which in my case is a different class (i.e. my test class extending KernelTestcase):

https://github.com/symfony/framework-bundle/blob/3cbca8f6b93681102cab0fe0e3c23d16e4d0ded3/Test/KernelTestCase.php#L80-L83

How to reproduce

use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;

class MyTest extends WebTestCase
{
    protected static $kernel;

    public function testUserlist()
    {
        static::createClient();
   }
}

Possible Solution

change all occurences to either self or static

Additional Context

No response