tests: shut down node after decom in nodes_decommissioning_test

Once we improve redpanda to make decommed nodes stop serving
Kafka traffic, this won't be necessary.  Right now, redpanda
does require that decommed nodes are stopped in order to avoid
confusing clients, so the test should do that.

Fixes #3277

tests/rptest/tests/nodes_decommissioning_test.py

@@ -132,6 +132,13 @@ def test_decommissioning_working_node(self):
             timeout_sec=120,
             backoff_sec=2)
 
+        # Stop the decommissioned node, because redpanda internally does not
+        # fence it, it is the responsibility of external orchestrator to
+        # stop the node they intend to remove.
+        # This can be removed when we change redpanda to prevent decommissioned nodes
+        # from responding to client Kafka requests.
+        self.redpanda.stop_node(to_decommission)
+
         self.run_validation(enable_idempotence=False, consumer_timeout_sec=45)
 
     @cluster(num_nodes=6, log_allow_list=CHAOS_LOG_ALLOW_LIST)