fix fallout of dotnet/xharness#1388

Author: pavelsavara

src/tests/Common/XHarnessRunnerLibrary/GeneratedTestRunner.cs

@@ -48,14 +48,14 @@ public override Task Run(IEnumerable<TestAssemblyInfo> testAssemblies)
         return Task.CompletedTask;
     }
 
-    public override string WriteResultsToFile(XmlResultJargon xmlResultJargon)
+    public override Task<string> WriteResultsToFile(XmlResultJargon xmlResultJargon)
     {
         Debug.Assert(xmlResultJargon == XmlResultJargon.xUnit);
         File.WriteAllText(ResultsFileName, LastTestRun.GetTestResultOutput(_assemblyName));
-        return ResultsFileName;
+        return Task.FromResult(ResultsFileName);
     }
 
-    public override void WriteResultsToFile(TextWriter writer, XmlResultJargon jargon)
+    public override Task WriteResultsToFile(TextWriter writer, XmlResultJargon jargon)
     {
         Debug.Assert(jargon == XmlResultJargon.xUnit);
         string lastTestResults = LastTestRun.GetTestResultOutput(_assemblyName);
@@ -69,6 +69,7 @@ public override void WriteResultsToFile(TextWriter writer, XmlResultJargon jargo
         {
             writer.WriteLine(lastTestResults);
         }
+        return Task.CompletedTask;
     }
 
     public override void SkipTests(IEnumerable<string> tests)