I have no idea about how Unity works, but perhaps instead of writing your behaviour in a class that extends monobehavior, you could create a pure-C# object and delegate all interesting stuff to it.
How do you make classes that extend monobehavior testable? Or maybe I'm just not far enough into the project where I can use composition on those bits.
Avi - nice to see you're still out there :-) I've played around a bit with Unity and took the weak position vis-¨¤-vis TDD.
That is: the visual interface is tested by looking at it and the rest is just normal C# code with normal tests.