[JointStateBroadcaster] Add tests for clearing joint_names in JointStateBroadcaster

[destogl]

Two cases should be tested:

1. When broadcaster is reactivated, joints should not be duplicated.
2. When broadcaster is deactivated, the joint names should be cleaned.

That's technically be more correct, but I think it's even more bulletproof to let the function generating a new setup do a quick cleanup -- as-is now

Originally posted by @bmagyar in https://github.com/ros-controls/ros2_controllers/pull/431#discussion_r963292043