NUnit integration

https://github.com/OpenRA/OpenRA/issues/3268
`./` is valid everywhere, including windows.  We use `./` for defining paths in our mod.yaml, but the (os-preferred) system separator in most of the engine code.  I would normally suggest that you use `Path.DirectorySeparatorChar`, but I think that will then give inconsistent paths under windows (if the rest of the path is using `/`).