auth_SUITE: Declare test auth backend module

[Why]
rabbit_access_control now ensures all configured auth backend modules
are provided by `rabbit` or an enabled plugin. This is not the case for
this testsuite's own auth backend.

[How]
Declare `rabbit_auth_backend_mqtt_mock` as a test auth backend module.

Author: dumbbell

deps/rabbitmq_mqtt/test/auth_SUITE.erl

@@ -251,7 +251,8 @@ auth_config(T) when T == client_id_propagation;
                     T == ssl_user_with_client_id_in_cert_san_email;
                     T == ssl_user_with_client_id_in_cert_dn ->
     {rabbit, [
-            {auth_backends, [rabbit_auth_backend_mqtt_mock]}
+            {auth_backends, [rabbit_auth_backend_mqtt_mock]},
+            {test_auth_backends, [rabbit_auth_backend_mqtt_mock]}
           ]
     };