Improve test failures for unexpected logs (#31406)

* Add ToString() to Testing's log WriteContext

* Update StrictTestServerTests to only show unexpected log

Author: halter73

src/Servers/IIS/IIS/test/IIS.Tests/StrictTestServerTests.cs

@@ -1,11 +1,13 @@
 // 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.Linq;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Testing;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Testing;
 using Xunit;
+using Xunit.Sdk;
 
 namespace Microsoft.AspNetCore.Server.IIS.FunctionalTests
 {
@@ -14,7 +16,11 @@ public class StrictTestServerTests: LoggedTest
         public override void Dispose()
         {
             base.Dispose();
-            Assert.DoesNotContain(TestSink.Writes, w => w.LogLevel > LogLevel.Information);
+
+            if (TestSink.Writes.FirstOrDefault(w => w.LogLevel > LogLevel.Information) is WriteContext writeContext)
+            {
+                throw new XunitException($"Unexpected log: {writeContext}");
+            }
         }
 
         protected static TaskCompletionSource<bool> CreateTaskCompletionSource()

src/Testing/src/Logging/WriteContext.cs

@@ -29,5 +29,10 @@ public string Message
                 return Formatter(State, Exception);
             }
         }
+
+        public override string ToString()
+        {
+            return $"{LogLevel} {LoggerName}: {Message}";
+        }
     }
-}
+}