Updated the C++ unit tests to NUnit 3.0

## Do Not Merge ##

Fixes nunit/nunit#234

I am still working on this. See PR nunit/nunit#414 for more information.

This is currently referencing `nunit.framework.dll` that was built in the nunit project. C++ projects cannot use managed NuGet packages, so I will put a copy of `nunit.framework.dll` in a `lib` directory when I am done making changes to the main project for better CLS compliance.