flaky test fix - assert cluster stats on isolated node as stats is local to the node (#13383)

Signed-off-by: Rajiv Kumar Vaidyanathan <rajivkv@amazon.com>

Author: rajiv-kv

server/src/internalClusterTest/java/org/opensearch/discovery/ClusterManagerDisruptionIT.java

@@ -198,11 +198,15 @@ public void testIsolateClusterManagerAndVerifyClusterStateConsensus() throws Exc
                             + nodeState
                     );
                 }
-
             }
-            ClusterStateStats clusterStateStats = internalCluster().clusterService().getClusterManagerService().getClusterStateStats();
-            assertTrue(clusterStateStats.getUpdateFailed() > 0);
+
         });
+
+        ClusterStateStats clusterStateStats = internalCluster().clusterService(isolatedNode)
+            .getClusterManagerService()
+            .getClusterStateStats();
+        assertTrue(clusterStateStats.getUpdateFailed() > 0);
+
     }
 
     /**