The aim is to build a relatively simple (2D) game in Unity with most of the game logic in a non-visual AI. The AI needs a lot of testing and debugging independently of any visuals (which is best done using VS tools), and the API connecting them is one class and a few methods/properties. [A future version might not even use Unity.]
So the question is how to set up a VS solution file to build the AI for testing and debugging while remaining compatible with the Mono build environment used by Unity. Yes, I know it has to be C# 4, but beyond that? At present I'm copying CS files around and that doesn't feel right.
Just to make it crystal clear, the AI has no dependency on Unity and will be built in a project that has VS testing and code analysis but no Unity tools or imports. In future it may be used with a different engine.