Add kestrel tests.

Tests will 'build', 'run', 'publish' and 'execute' a Kestrel Hello World server as a PortableFatApp and as a Standalone app.

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

@@ -0,0 +1,90 @@
+using System;
+using System.Collections.Generic;
+using System.Net;
+using System.Net.Http;
+using System.Net.Sockets;
+using System.Runtime.InteropServices;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.DotNet.ProjectModel;
+using FluentAssertions;
+
+namespace Microsoft.DotNet.Tools.Test.Utilities
+{
+    public class NetworkHelper
+    {
+        // in milliseconds
+        private const int Timeout = 20000;
+
+        private static Queue<TcpListener> s_PortPool = new Queue<TcpListener>();
+
+        public static string Localhost { get; } = "http://localhost";
+
+        public static bool IsServerUp(string url)
+        {
+            return SpinWait.SpinUntil(() =>
+            {
+                using (var client = new HttpClient())
+                {
+                    try
+                    {
+                        client.BaseAddress = new Uri(url);
+                        HttpResponseMessage response = client.GetAsync("").Result;
+                        return response.IsSuccessStatusCode;
+                    }
+                    catch (Exception)
+                    {
+                        return false;
+                    }
+                }
+            }, Timeout);
+        }
+
+        public static void TestGetRequest(string url, string expectedResponse)
+        {
+            using (var client = new HttpClient())
+            {
+                client.BaseAddress = new Uri(url);
+
+                HttpResponseMessage response = client.GetAsync("").Result;
+                if (response.IsSuccessStatusCode)
+                {
+                    var responseString = response.Content.ReadAsStringAsync().Result;
+                    responseString.Should().Contain(expectedResponse);
+                }
+            }
+        }
+
+        public static int GetFreePort()
+        {
+            lock (s_PortPool)
+            {
+                if (s_PortPool.Count == 0)
+                {
+                    for (int i = 0; i < 20; i++)
+                    {
+                        var tcpl = new TcpListener(IPAddress.Loopback, 0);
+                        tcpl.Start();
+                        s_PortPool.Enqueue(tcpl);
+                    }
+
+                    Console.WriteLine($"Ports Count >>>>>>>>>>>>>>>>>>> {s_PortPool.Count}");
+                }
+
+                var currentTcpl = s_PortPool.Dequeue();
+                var port = ((IPEndPoint)currentTcpl.LocalEndpoint).Port;
+                currentTcpl.Stop();
+                currentTcpl = null;
+                GC.Collect();
+                GC.WaitForPendingFinalizers();
+
+                return port;
+            }
+        }
+
+        public static string GetLocalhostUrlWithFreePort()
+        {
+            return $"{Localhost}:{GetFreePort()}/";
+        }
+    }
+}