Added missing file header.

Author: Matheus Inácio

UnitTests/UnitTests.UWP/Extensions/Test_DispatcherQueueTimerExtensions.cs

@@ -1,3 +1,7 @@
+// 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 Microsoft.Toolkit.Uwp.UI;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using System;