mqtt_shared_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.

deps/rabbitmq_mqtt/test/mqtt_shared_SUITE.erl

@@ -204,8 +204,13 @@ init_per_group(Group, Config0) ->
                 Config,
                 rabbit_ct_broker_helpers:setup_steps() ++
                     rabbit_ct_client_helpers:setup_steps()),
-    util:enable_plugin(Config1, rabbitmq_mqtt),
-    Config1.
+    case Config1 of
+        _ when is_list(Config1) ->
+            util:enable_plugin(Config1, rabbitmq_mqtt),
+            Config1;
+        {skip, _} ->
+            Config1
+    end.
 
 end_per_group(G, Config)
   when G =:= cluster_size_1;