Do not exclude AdminRouterSubscriberTest from test suite & rewrite un…

[michaelKaefer]

…used ExceptionListenerTest which would not work anymore & add a conflict to composer.json to fix method signature missmatch

Note on the conflict in composer.json: the following method signature is not compatible with symfony/error-handler lower than 5.4.35 and throws a fatal error if a lower version is used:

final class FlattenException extends BaseFlattenException
{
    public static function create(\Exception $exception, ?int $statusCode = null, array $headers = []): static

[javiereguiluz]

Michael, thanks so much for this one 🙇 I remember trying to fix this and I couldn't and it frustrated me so much 😢