Add Missing Header

Author: michael-hawker

UnitTests/UnitTests.UWP/Extensions/Test_LogicalTreeExtensions.cs

@@ -1,12 +1,12 @@
-using Microsoft.Toolkit.Uwp.Extensions;
-using Microsoft.Toolkit.Uwp.UI.Extensions;
-using Microsoft.VisualStudio.TestTools.UnitTesting;
-using Microsoft.VisualStudio.TestTools.UnitTesting.AppContainer;
-using System;
-using System.Collections.Generic;
+// Licensed to the .NET Foundation under one or more agreements.
+// 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.Linq;
-using System.Text;
 using System.Threading.Tasks;
+using Microsoft.Toolkit.Uwp.Extensions;
+using Microsoft.Toolkit.Uwp.UI.Extensions;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
 using Windows.UI.Xaml;
 using Windows.UI.Xaml.Controls;
 using Windows.UI.Xaml.Markup;