Add missing license headers

- #EngineeringDay
- skip generated files such as Resources.Designer.cs and files under
  test\Microsoft.AspNet.Razor.Test\TestFiles\CodeGenerator\CS\Output

test/Microsoft.AspNet.Razor.Test/Utils/BaselineWriter.cs

@@ -1,4 +1,7 @@
-using System.Diagnostics;
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Diagnostics;
 using System.IO;
 
 namespace Microsoft.AspNet.Razor.Test.Utils

test/Microsoft.AspNet.Razor.Test/Utils/ExceptionHelpers.cs

@@ -1,4 +1,7 @@
-using System;
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
 using Xunit;
 
 namespace Microsoft.AspNet.Razor.Test