Add tests for hardware_spawner and other python scripts

[christophfroehlich]

With https://github.com/ros-controls/ros2_control/pull/1668 we have now coverage build also for the python scripts. some of the files have zero or low coverage: https://app.codecov.io/gh/ros-controls/ros2_control/pull/1668/tree/controller_manager/controller_manager

Maybe there is some dead code and we should remove it at all.