Move test-URDF from transmission interface to `ros2_control_test_assets`

[destogl]

I propose to extend the minimal robot from the ros2_control_test_assets with transmission tags too. This should be then used for testing of transmissions.

The idea is to have one fully-featured URDF for doing main test of different features. Although not all tags will be used in all tests, they all have to be parsable and should not collide with each other. This can be only achieved if we really use the same test-URDF in most cases. Moreover, this reduces code duplication and potential issues is URDF get updated anytime in the future.

Original comment:

Why not to add this into ros2_control_test_assets? It would be a better and reusable place for this. Maybe there is already exactly the one you need. Or perhaps we can only extend the existing MinimalRobot with this.

Originally posted by @destogl in #633 (comment)