[Notifier] Replace tests dummy instances by already in place mocks

Author: alexandre-daubois

Tests/GatewayApiTransportTest.php

@@ -17,8 +17,7 @@
 use Symfony\Component\Notifier\Message\SentMessage;
 use Symfony\Component\Notifier\Message\SmsMessage;
 use Symfony\Component\Notifier\Test\TransportTestCase;
-use Symfony\Component\Notifier\Tests\Fixtures\DummyHttpClient;
-use Symfony\Component\Notifier\Tests\Fixtures\DummyMessage;
+use Symfony\Component\Notifier\Tests\Transport\DummyMessage;
 use Symfony\Component\Notifier\Transport\TransportInterface;
 use Symfony\Contracts\HttpClient\HttpClientInterface;
 use Symfony\Contracts\HttpClient\ResponseInterface;
@@ -34,7 +33,7 @@ final class GatewayApiTransportTest extends TransportTestCase
      */
     public static function createTransport(HttpClientInterface $client = null): TransportInterface
     {
-        return new GatewayApiTransport('authtoken', 'Symfony', $client ?? new DummyHttpClient());
+        return new GatewayApiTransport('authtoken', 'Symfony', $client ?? new MockHttpClient());
     }
 
     public static function toStringProvider(): iterable