Refactored RobotModel test code to be parameterized gtest

Pulled `RobotModel` interface test into `descartes_core`.  Tests can not be pulled into any implementation of `RobotModel` (`descartes_moveit` and others).
@dpsolomon, see the comment above this declaration.  Does this sound familiar?  This issues shouldn't hold up this PR (it's in test code), but I could not figure it out for the life of me.