Use port 0 to avoid port collisions during tests

test/Microsoft.AspNetCore.WebSockets.Test/IWebHostPortExtensions.cs

+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.AspNetCore.Hosting.Server.Features;
+
+namespace Microsoft.AspNetCore.Hosting
+{
+    public static class IWebHostPortExtensions
+    {
+        public static int GetPort(this IWebHost host)
+        {
+            return host.GetPorts().First();
+        }
+
+        public static IEnumerable<int> GetPorts(this IWebHost host)
+        {
+            return host.GetUris()
+                .Select(u => u.Port);
+        }
+
+        public static IEnumerable<Uri> GetUris(this IWebHost host)
+        {
+            return host.ServerFeatures.Get<IServerAddressesFeature>().Addresses
+                .Select(a => new Uri(a));
+        }
+    }
+}
\ No newline at end of file

test/Microsoft.AspNetCore.WebSockets.Test/KestrelWebSocketHelpers.cs

@@ -14,7 +14,7 @@ namespace Microsoft.AspNetCore.WebSockets.Test
 {
     public class KestrelWebSocketHelpers
     {
-        public static IDisposable CreateServer(ILoggerFactory loggerFactory, Func<HttpContext, Task> app, int port)
+        public static IDisposable CreateServer(ILoggerFactory loggerFactory, out int port, Func<HttpContext, Task> app)
         {
             Action<IApplicationBuilder> startup = builder =>
             {
@@ -45,7 +45,7 @@ namespace Microsoft.AspNetCore.WebSockets.Test
             var configBuilder = new ConfigurationBuilder();
             configBuilder.AddInMemoryCollection();
             var config = configBuilder.Build();
-            config["server.urls"] = $"http://localhost:{port}";
+            config["server.urls"] = $"http://127.0.0.1:0";
 
             var host = new WebHostBuilder()
                 .ConfigureServices(s => s.AddSingleton(loggerFactory))
@@ -55,6 +55,7 @@ namespace Microsoft.AspNetCore.WebSockets.Test
                 .Build();
 
             host.Start();
+            port = host.GetPort();
 
             return host;
         }