Use ToolsVersion 4 for TestHelper
Use ToolsVersion 4 for TestHelper

This allows to compile with Mono 3.x which we currently have on some
TC build agents.

ermshiperete/icu-dotnet,sillsdev/icu-dotnet,sillsdev/icu-dotnet,conniey/icu-dotnet,ermshiperete/icu-dotnet,conniey/icu-dotnet