Added missing file headers

Author: Arlodotexe

CommunityToolkit.Tooling.SampleGen.Tests/Helpers/InMemoryAdditionalText.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.CodeAnalysis;
 using Microsoft.CodeAnalysis.Text;
 

CommunityToolkit.Tooling.SampleGen.Tests/Helpers/TestHelpers.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.CodeAnalysis;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using System.Collections.Immutable;