Fix broken unit tests

Tests/MQTTnet.Core.Tests/MqttClientTests.cs

@@ -0,0 +1,37 @@
+using System;
+using System.Net.Sockets;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Client;
+using MQTTnet.Exceptions;
+
+namespace MQTTnet.Core.Tests
+{
+    [TestClass]
+    public class MqttClientTests
+    {
+
+        [TestMethod]
+        public async Task ClientDisconnectException()
+        {
+            var factory = new MqttFactory();
+            var client = factory.CreateMqttClient();
+
+            var exceptionIsCorrect = false;
+            client.Disconnected += (s, e) =>
+            {
+                exceptionIsCorrect = e.Exception is MqttCommunicationException c && c.InnerException is SocketException;
+            };
+
+            try
+            {
+                await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("wrong-server").Build());
+            }
+            catch
+            {
+            }
+            
+            Assert.IsTrue(exceptionIsCorrect);
+        }
+    }
+}