Fix build error: add missing System.Linq using directive

Fix compilation error on line 336 where OrderBy LINQ extension method
was not accessible on AsyncEvent<TestContext> type.

Co-authored-by: Tom Longhurst <thomhurst@users.noreply.github.com>

Author: claude[bot]

TUnit.Engine/Services/SingleTestExecutor.cs

@@ -1,3 +1,4 @@
+using System.Linq;
 using System.Runtime.ExceptionServices;
 using Microsoft.Testing.Platform.Extensions.Messages;
 using Microsoft.Testing.Platform.TestHost;