Fix AddressInUseException in tests

BTCPayServer.Tests/Utils.cs

@@ -10,13 +10,34 @@ namespace BTCPayServer.Tests
 {
     public class Utils
     {
+        public static int _nextPort = 8001;
+        public static object _portLock = new object();
+
         public static int FreeTcpPort()
         {
-            TcpListener l = new TcpListener(IPAddress.Loopback, 0);
-            l.Start();
-            int port = ((IPEndPoint)l.LocalEndpoint).Port;
-            l.Stop();
-            return port;
+            lock (_portLock)
+            {
+                using (var socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp))
+                {
+                    while (true)
+                    {
+                        try
+                        {
+                            var port = _nextPort++;
+                            socket.Bind(new IPEndPoint(IPAddress.Loopback, port));
+                            return port;
+                        }
+                        catch (SocketException)
+                        {
+                            // Retry unless exhausted
+                            if (_nextPort == 65536)
+                            {
+                                throw;
+                            }
+                        }
+                    }
+                }
+            }
         }
 
         // http://stackoverflow.com/a/14933880/2061103