Remove `robot_descriptions` test dependency

[adlarkin]

Right now, I am using models from the robot_descriptions package for testing. The initial motivation was to reduce the amount of model files in the repo, but I think there are some drawbacks:

1. Models have to be downloaded every time CI is ran
2. It's hard to debug models if needed since the downloaded models are in a cache that's not a part of the repository
3. There's a risk of models changing upstream and breaking the current tests

I should probably just add whatever test models I need to this repo's test/models directory, even if that means duplication between example and test models (or maybe I can have a commonplace for models that are shared between examples and tests).