fix xharness

[pavelsavara]

fix fallout of dotnet/xharness#1388

[Copilot]

Pull Request Overview

This PR aims to update the XHarness test runner to use asynchronous return types for writing test results in response to fallout from a previous change.

• Updated WriteResultsToFile(XmlResultJargon) to return Task instead of a synchronous string.
• Updated WriteResultsToFile(TextWriter, XmlResultJargon) to return Task instead of void.

Comments suppressed due to low confidence (2)

src/tests/Common/XHarnessRunnerLibrary/GeneratedTestRunner.cs:55

• Although the method now returns a Task, it still performs synchronous file I/O. Consider using File.WriteAllTextAsync to fully support asynchronous operations.

File.WriteAllText(ResultsFileName, LastTestRun.GetTestResultOutput(_assemblyName));

src/tests/Common/XHarnessRunnerLibrary/GeneratedTestRunner.cs:72

• The synchronous write call may block the execution thread even though the method returns a Task. If the TextWriter supports asynchronous operations, consider using WriteLineAsync.

writer.WriteLine(lastTestResults);