auth_SUITE: Handle error returned by rabbit_ct_broker_helpers

[Why]
It didn't handle them before and crashed later when it assumed the
return value was a list.

Author: dumbbell

deps/rabbitmq_mqtt/test/auth_SUITE.erl

@@ -185,8 +185,13 @@ init_per_group(Group, Config) ->
        end] ++
           rabbit_ct_broker_helpers:setup_steps() ++
           rabbit_ct_client_helpers:setup_steps()),
-    util:enable_plugin(Config2, rabbitmq_mqtt),
-    Config2.
+    case Config2 of
+        _ when is_list(Config2) ->
+            util:enable_plugin(Config2, rabbitmq_mqtt),
+            Config2;
+        {skip, _} ->
+            Config2
+    end.
 
 end_per_group(G, Config)
   when G =:= v4;