Get free port starting from 8001 instead of using TcpListener.

Enable kestrel tests to run in parallel.

test/Microsoft.DotNet.Tools.Tests.Utilities/NetworkUtils/PortManager.cs

@@ -0,0 +1,16 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+
+namespace Microsoft.DotNet.Tools.Test.Utilities
+{
+    public static class PortManager
+    {
+        private static int s_nextPort = 8001;
+
+        public static int GetPort()
+        {
+            return Interlocked.Increment(ref s_nextPort);
+        }
+    }
+}