[Messenger] Add hint test config (#1084)

* Add test config

Add hint for test config

* Change comment location and add testing tools for Messenger

symfony/messenger/6.0/config/packages/messenger.yaml

@@ -12,3 +12,11 @@ framework:
         routing:
             # Route your messages to the transports
             # 'App\Message\YourMessage': async
+
+# when@test:
+#    framework:
+#        messenger:
+#            transports:
+#                # replace with your transport name here (e.g., my_transport: 'in-memory://')
+#                # For more Messenger testing tools, see https://github.com/zenstruck/messenger-test
+#                async: 'in-memory://'