Fixed build.

Tests/MQTTnet.Core.Tests/ManagedMqttClient_Tests.cs

@@ -1,4 +1,5 @@
-using System.Threading;
+using System.Collections.Generic;
+using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;