OF-2437: Introduce CachingPubsubPersistenceProviderTest

This new unit test verifies behavior of CachingPubsubPersistenceProvider (which fails, as indicated by some of these test failing).

The tests introduced here only cover one (of four) types of operations that are being scheduled by the implementation:
- Node creation/changing/removal

These two other types of operation still need to be added:
- Affiliation creation/update/removal
- Subscription creation/update/removal
- publishing/removal of items

xmppserver/src/main/java/org/jivesoftware/openfire/pubsub/CachingPubsubPersistenceProvider.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2020-2022 Ignite Realtime Foundation. All rights reserved.
+ * Copyright (C) 2020-2025 Ignite Realtime Foundation. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -15,6 +15,7 @@
  */
 package org.jivesoftware.openfire.pubsub;
 
+import com.google.common.annotations.VisibleForTesting;
 import org.jivesoftware.openfire.cluster.ClusterManager;
 import org.jivesoftware.openfire.pep.PEPService;
 import org.jivesoftware.openfire.pubsub.cluster.FlushTask;
@@ -69,20 +70,24 @@ public class CachingPubsubPersistenceProvider implements PubSubPersistenceProvid
     /**
      * Queue that holds the (wrapped) items that need to be added to the database.
      */
-    private Deque<PublishedItem> itemsToAdd = new ConcurrentLinkedDeque<>();
+    @VisibleForTesting
+    Deque<PublishedItem> itemsToAdd = new ConcurrentLinkedDeque<>();
 
     /**
      * Queue that holds the items that need to be deleted from the database.
      */
-    private Deque<PublishedItem> itemsToDelete = new ConcurrentLinkedDeque<>();
+    @VisibleForTesting
+    Deque<PublishedItem> itemsToDelete = new ConcurrentLinkedDeque<>();
 
     /**
      * Keeps reference to published items that haven't been persisted yet so they
      * can be removed before being deleted.
      */
-    private final HashMap<PublishedItem.UniqueIdentifier, PublishedItem> itemsPending = new HashMap<>();
+    @VisibleForTesting
+    final HashMap<PublishedItem.UniqueIdentifier, PublishedItem> itemsPending = new HashMap<>();
 
-    private ConcurrentMap<Node.UniqueIdentifier, List<NodeOperation>> nodesToProcess = new ConcurrentHashMap<>();
+    @VisibleForTesting
+    final ConcurrentMap<Node.UniqueIdentifier, List<NodeOperation>> nodesToProcess = new ConcurrentHashMap<>();
 
     /**
      * Cache name for recently accessed published items.