Reduce probability of MultipleAppTests failure (#1365)

- Decrease false-positive failure rate from 1 in 1,000 to 1 in 1,000,000
- Addresses #1350

test/IISExpress.FunctionalTests/OutOfProcess/MultipleAppTests.cs

@@ -1,6 +1,7 @@
 // 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.Net;
 using System.Net.Http;
@@ -25,7 +26,19 @@ namespace Microsoft.AspNetCore.Server.IISIntegration.FunctionalTests
         [ConditionalTheory]
         [InlineData(AncmVersion.AspNetCoreModule)]
         [InlineData(AncmVersion.AspNetCoreModuleV2)]
-        public async Task Startup(AncmVersion ancmVersion)
+        public Task Startup(AncmVersion ancmVersion)
+        {
+            // ANCM v1 currently does *not* retry if an app fails to start the first time due to a port collision.
+            // So, this test is expected to fail on v1 approximately 1 in 1,000 times (probably of at least one collision
+            // when 10 sites choose a random port from the range 1025-48000).  Adding one retry should reduce the failure
+            // rate from 1 in 1,000 to 1 in 1,000,000.  The previous product code (with "srand(GetTickCount())") should still
+            // fail the test reliably.
+            // https://github.com/aspnet/IISIntegration/issues/1350
+            // 
+            // ANCM v2 does retry on port collisions, so no retries should be required.
+            var attempts = (ancmVersion == AncmVersion.AspNetCoreModule) ? 2 : 1;
+
+            return Retry(async () =>
             {
                 const int numApps = 10;
 
@@ -61,6 +74,28 @@ namespace Microsoft.AspNetCore.Server.IISIntegration.FunctionalTests
                         Assert.Equal("Hello World", responseText);
                     }
                 }
+            },
+            attempts: attempts);
+        }
+
+        private async Task Retry(Func<Task> func, int attempts)
+        {
+            var exceptions = new List<Exception>();
+
+            for (var attempt = 0; attempt < attempts; attempt++)
+            {
+                try
+                {
+                    await func();
+                    return;
+                }
+                catch (Exception e)
+                {
+                    exceptions.Add(e);
+                }
+            }
+
+            throw new AggregateException(exceptions);
         }
     }
 }