Add using directive for System.Runtime.InteropServices

Added a new using directive for the System.Runtime.InteropServices
namespace in the TestHelper.cs file to enable the use of its types
and methods.

Author: Aditya Narayan

src/TestUtilities/TestHelper.cs

@@ -8,6 +8,7 @@
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.IO;
+using System.Runtime.InteropServices;
 using System.Text;
 
 namespace TestUtilities