Merge branch '6.2' into 6.3

* 6.2:
  [Notifier] Replace tests dummy instances by already in place mocks
  Use PHPUnit 9.6 to run Symfony's test suite
  [DependencyInjection] Add doc for RUNTIME_EXCEPTION_ON_INVALID_REFERENCE behavior

Author: nicolas-grekas

Tests/LinkedInTransportTest.php

@@ -16,22 +16,19 @@
 use Symfony\Component\Notifier\Exception\LogicException;
 use Symfony\Component\Notifier\Exception\TransportException;
 use Symfony\Component\Notifier\Message\ChatMessage;
-use Symfony\Component\Notifier\Message\MessageInterface;
 use Symfony\Component\Notifier\Message\MessageOptionsInterface;
 use Symfony\Component\Notifier\Message\SmsMessage;
 use Symfony\Component\Notifier\Notification\Notification;
 use Symfony\Component\Notifier\Test\TransportTestCase;
-use Symfony\Component\Notifier\Tests\Fixtures\DummyHttpClient;
-use Symfony\Component\Notifier\Tests\Fixtures\DummyMessage;
-use Symfony\Component\Notifier\Transport\TransportInterface;
+use Symfony\Component\Notifier\Tests\Transport\DummyMessage;
 use Symfony\Contracts\HttpClient\HttpClientInterface;
 use Symfony\Contracts\HttpClient\ResponseInterface;
 
 final class LinkedInTransportTest extends TransportTestCase
 {
     public static function createTransport(HttpClientInterface $client = null): LinkedInTransport
     {
-        return (new LinkedInTransport('AuthToken', 'AccountId', $client ?? new DummyHttpClient()))->setHost('host.test');
+        return (new LinkedInTransport('AuthToken', 'AccountId', $client ?? new MockHttpClient()))->setHost('host.test');
     }
 
     public static function toStringProvider(): iterable