Fix error when passing in a null run settings value on .net 6 SDKs

src/Features/LanguageServer/Microsoft.CodeAnalysis.LanguageServer/Testing/TestRunner.cs

@@ -19,6 +19,13 @@ namespace Microsoft.CodeAnalysis.LanguageServer.Testing;
 [method: Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
 internal partial class TestRunner(ILoggerFactory loggerFactory)
 {
+    /// <summary>
+    /// A default value for run settings.  While the vstest console included with newer SDKs does
+    /// support passing in a null run settings value, the vstest console in older SDKs (.net 6 for example)
+    /// will throw if we pass a null value.  So for our default we hardcode an empty XML configuration.
+    /// TODO - Support configuring run settings - https://github.com/dotnet/vscode-csharp/issues/5719
+    /// </summary>
+    private static readonly string s_defaultRunSettings = "<RunSettings/>";
     private readonly ILogger _logger = loggerFactory.CreateLogger<TestRunner>();
 
     public async Task RunTestsAsync(
@@ -53,13 +60,12 @@ private static void RunTests(
         if (attachDebugger)
         {
             // When we want to debug tests we need to use a custom test launcher so that we get called back with the process to attach to.
-            vsTestConsoleWrapper.RunTestsWithCustomTestHost(testCases, runSettings: null, handler, new DebugTestHostLauncher(progress, clientLanguageServerManager));
+            vsTestConsoleWrapper.RunTestsWithCustomTestHost(testCases, runSettings: s_defaultRunSettings, handler, new DebugTestHostLauncher(progress, clientLanguageServerManager));
         }
         else
         {
             // The async APIs for vs test are broken (current impl ends up just hanging), so we must use the sync API instead.
-            // TODO - run settings. https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1799066/
-            vsTestConsoleWrapper.RunTests(testCases, runSettings: null, handler);
+            vsTestConsoleWrapper.RunTests(testCases, runSettings: s_defaultRunSettings, handler);
         }
     }
 }