Improve unit tests

Source/MQTTnet.Tests/Helpers/MqttClientExtensions.cs

+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using MQTTnet.Client;
+using MQTTnet.Tests.Mockups;
+
+namespace MQTTnet.Tests.Helpers
+{
+    public static class MqttClientExtensions
+    {
+        public static TestApplicationMessageReceivedHandler TrackReceivedMessages(this IMqttClient client)
+        {
+            if (client == null)
+            {
+                throw new ArgumentNullException(nameof(client));
+            }
+
+            return new TestApplicationMessageReceivedHandler(client);
+        }
+    }
+}
\ No newline at end of file

Source/MQTTnet.Tests/Mockups/TestApplicationMessageReceivedHandler.cs

@@ -27,6 +27,17 @@ namespace MQTTnet.Tests.Mockups
             mqttClient.ApplicationMessageReceivedAsync += OnApplicationMessageReceivedAsync;
         }
 
+        public int Count
+        {
+            get
+            {
+                lock (_receivedEventArgs)
+                {
+                    return _receivedEventArgs.Count;
+                }
+            }
+        }
+
         public List<MqttApplicationMessageReceivedEventArgs> ReceivedEventArgs
         {
             get