Use files from demos for testing (backport #485)

[mergify]

Use the demos directly instead of having different launch description, URDF, and executables for testing. (I just copied the content of the test_position executable to the example_position file in the demos).

I also added separate tests for the clock topic (sim_time), and if the expected controllers are loaded.

velocity and effort tests currently don't really test the system, because no feedback is processed. I'll do that in a new PR because it will change the demos itself.

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This is an automatic backport of pull request #485 done by Mergify.

[mergify]

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Changes to be committed:
	modified:   gz_ros2_control_tests/CMakeLists.txt
	deleted:    gz_ros2_control_tests/src/test_position.cpp
	modified:   gz_ros2_control_tests/tests/CMakeLists.txt
	new file:   gz_ros2_control_tests/tests/effort_test.py
	new file:   gz_ros2_control_tests/tests/velocity_test.py
	modified:   ign_ros2_control_demos/examples/example_position.cpp
	modified:   ign_ros2_control_demos/launch/ackermann_drive_example.launch.py
	modified:   ign_ros2_control_demos/launch/pendulum_example_effort.launch.py
	modified:   ign_ros2_control_demos/urdf/test_cart_velocity.xacro.urdf

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by them: gz_ros2_control_tests/config/cart_controller_position.yaml
	both modified:   gz_ros2_control_tests/package.xml
	both modified:   gz_ros2_control_tests/tests/position_test.py
	deleted by them: gz_ros2_control_tests/urdf/test_cart_position.xacro.urdf
	both modified:   ign_ros2_control_demos/launch/diff_drive_example_namespaced.launch.py
	both modified:   ign_ros2_control_demos/urdf/test_cart_position.xacro.urdf

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