Enable NativeAOT support for iOS test runner (#1554)

Make ThreadlessXunitTestRunner platform-generic so it can be used on
iOS/tvOS/Mac Catalyst in addition to WASM. The runner uses reflection-based
test discovery (ThreadlessXunitDiscoverer) which is NativeAOT-compatible,
unlike XUnitTestRunner which depends on XunitFrontController for dynamic
assembly loading.

Changes:
- ThreadlessXunitTestRunner: make ResultsFileName settable, refactor
  WriteResultsToFile to support file-based XML output on non-browser
  platforms while preserving existing WASM behavior
- iOSApplicationEntryPoint: detect NativeAOT via
  RuntimeFeature.IsDynamicCodeSupported and automatically use
  ThreadlessXunitTestRunner when dynamic code is not available

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: kotlarmilos

src/Microsoft.DotNet.XHarness.TestRunners.Xunit/ThreadlessXunitTestRunner.cs

@@ -8,6 +8,7 @@
 using System.IO;
 using System.Linq;
 using System.Threading.Tasks;
+using System.Xml;
 using System.Xml.Linq;
 using Microsoft.DotNet.XHarness.Common;
 using Microsoft.DotNet.XHarness.TestRunners.Common;
@@ -24,7 +25,8 @@ public ThreadlessXunitTestRunner(LogWriter logger) : base(logger)
         ShowFailureInfos = false;
     }
 
-    protected override string ResultsFileName { get => string.Empty; set => throw new InvalidOperationException("This runner outputs its results to stdout."); }
+    private string _resultsFileName = "TestResults.xUnit.xml";
+    protected override string ResultsFileName { get => _resultsFileName; set => _resultsFileName = value; }
 
     private XElement? _assembliesElement;
 
@@ -121,14 +123,41 @@ private ExecutionSummary Combine(ExecutionSummary aggregateSummary, ExecutionSum
 
     public override async Task<string> WriteResultsToFile(XmlResultJargon xmlResultJargon)
     {
-        Debug.Assert(xmlResultJargon == XmlResultJargon.xUnit);
-        await WriteResultsToFile(Console.Out, xmlResultJargon);
-        return "";
+        if (_assembliesElement is null)
+            return string.Empty;
+
+        if (OperatingSystem.IsBrowser())
+        {
+            Debug.Assert(xmlResultJargon == XmlResultJargon.xUnit);
+            await WriteResultsToFile(Console.Out, xmlResultJargon);
+            return "";
+        }
+
+        string outputFilePath = GetResultsFilePath();
+        var settings = new XmlWriterSettings { Indent = true };
+        using (var xmlWriter = XmlWriter.Create(outputFilePath, settings))
+        {
+            _assembliesElement.Save(xmlWriter);
+        }
+
+        return outputFilePath;
     }
 
-    public override async Task WriteResultsToFile(TextWriter writer, XmlResultJargon jargon)
+    public override Task WriteResultsToFile(TextWriter writer, XmlResultJargon jargon)
     {
-        await WasmXmlResultWriter.WriteResultsToFile(ConsumeAssembliesElement());
+        if (_assembliesElement is null)
+            return Task.CompletedTask;
+
+        if (OperatingSystem.IsBrowser())
+            return WasmXmlResultWriter.WriteResultsToFile(ConsumeAssembliesElement());
+
+        var settings = new XmlWriterSettings { Indent = true };
+        using (var xmlWriter = XmlWriter.Create(writer, settings))
+        {
+            _assembliesElement.Save(xmlWriter);
+        }
+
+        return Task.CompletedTask;
     }
 }
 

src/Microsoft.DotNet.XHarness.TestRunners.Xunit/iOSApplicationEntryPoint.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Runtime.CompilerServices;
 using Microsoft.DotNet.XHarness.TestRunners.Common;
 
 #nullable enable
@@ -11,6 +12,13 @@ public abstract class iOSApplicationEntryPoint : iOSApplicationEntryPointBase
 {
     protected override TestRunner GetTestRunner(LogWriter logWriter)
     {
+        if (!RuntimeFeature.IsDynamicCodeSupported)
+        {
+            var threadlessRunner = new ThreadlessXunitTestRunner(logWriter);
+            ConfigureRunnerFilters(threadlessRunner, ApplicationOptions.Current);
+            return threadlessRunner;
+        }
+
         var runner = new XUnitTestRunner(logWriter) { MaxParallelThreads = MaxParallelThreads };
         ConfigureRunnerFilters(runner, ApplicationOptions.Current);
         return runner;