Add VSTHRD010 transitive test async methods

Repros #226

src/Microsoft.VisualStudio.Threading.Analyzers.Tests/VSTHRD010MainThreadUsageAnalyzerTests.cs

@@ -382,6 +382,32 @@ void VerifyOnUIThread() {
             this.VerifyCSharpDiagnostic(test, this.expect);
         }
 
+        [Fact]
+        public void RequiresUIThread_NotTransitiveThroughAsyncCalls()
+        {
+            var test = @"
+using System;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.Threading;
+
+class Test {
+    private JoinableTaskFactory jtf;
+
+    private void ShowToolWindow(object sender, EventArgs e) {
+        jtf.RunAsync(async delegate {
+            await FooAsync(); // this line is what adds the VSTHRD010 diagnostic
+        });
+    }
+
+    private async Task FooAsync() {
+        await jtf.SwitchToMainThreadAsync();
+    }
+}
+";
+
+            this.VerifyCSharpDiagnostic(test);
+        }
+
         [Fact]
         public void InvokeVsSolutionAfterSwitchedToMainThreadAsync()
         {