rabbit_mgmt_http_SUITE: Enable rabbitmq_4.0.0 feature flag

[Why]
This testsuite apparently requires it and if it's not enabled, it fails.

Author: dumbbell

deps/rabbitmq_management/test/rabbit_mgmt_http_SUITE.erl

@@ -244,7 +244,17 @@ start_broker(Config) ->
     Setup0 = rabbit_ct_broker_helpers:setup_steps(),
     Setup1 = rabbit_ct_client_helpers:setup_steps(),
     Steps = Setup0 ++ Setup1,
-    rabbit_ct_helpers:run_setup_steps(Config, Steps).
+    case rabbit_ct_helpers:run_setup_steps(Config, Steps) of
+        {skip, _} = Skip ->
+            Skip;
+        Config1 ->
+            Ret = rabbit_ct_broker_helpers:enable_feature_flag(
+                    Config1, 'rabbitmq_4.0.0'),
+            case Ret of
+                ok -> Config1;
+                _  -> Ret
+            end
+    end.
 
 finish_init(Group, Config) ->
     rabbit_ct_helpers:log_environment(),