c#: fix resource-alias-redux test to new framework

[jsturtevant]

          @jsturtevant I think that this is exposing a preexisting bug in the C# generator, I've added a test at `_test_disabled.cs` here which fails to compile in the generated code because I don't think it's `using` the right paths, but I could also very well be misunderstanding something about C# here in this respect.

Originally posted by @alexcrichton in #1250 (comment)