Add copyright

src/PluginsSystem/Microsoft.Performance.Toolkit.Plugins.Package.Tests/Usings.cs

@@ -1 +1,4 @@
-global using Microsoft.VisualStudio.TestTools.UnitTesting;
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License
+
+global using Microsoft.VisualStudio.TestTools.UnitTesting;