possibly fixed timing issue in test

MQTTnet.Core/Server/IMqttClientRetainedMessageManager.cs

@@ -0,0 +1,15 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using MQTTnet.Core.Packets;
+
+namespace MQTTnet.Core.Server
+{
+    public interface IMqttClientRetainedMessageManager
+    {
+        Task LoadMessagesAsync();
+
+        Task HandleMessageAsync(string clientId, MqttApplicationMessage applicationMessage);
+
+        List<MqttApplicationMessage> GetMessages(MqttSubscribePacket subscribePacket);
+    }
+}