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).
I thought `TearDown()` just overrode the `::testingTest` class function, so I have no idea why it would do this. I didn't see that you were doing any locks in your stuff, so I think it has to be a throw from the google test framework.