Explicitly match skips when setting metadata store in CT

This causes a clearer error when the `enable_feature_flags/2` function
returns something not in the shape `ok | {skip, any()}`.

Author: the-mikedavis

deps/rabbitmq_ct_helpers/src/rabbit_ct_broker_helpers.erl

@@ -992,7 +992,7 @@ enable_khepri_metadata_store(Config, FFs0) ->
                         case enable_feature_flag(C, FF) of
                             ok ->
                                 C;
-                            Skip ->
+                            {skip, _} = Skip ->
                                 ct:pal("Enabling metadata store failed: ~p", [Skip]),
                                 Skip
                         end