Add missing tests for pendulum and gripper (position and effort) (backport #814)

[mergify]

This PR adds several missing integration tests for the demos in gz_ros2_control_demos.

Included:

• pendulum position test
• gripper position test
• gripper effort test

These tests follow the same structure as the precedent tests:

• verify /joint_states is published
• verify controllers are loaded and active
• check initial joint state when applicable
• launch the example node and ensure it exits cleanly

Additional fixes:

• corrected launch files to accept gz_args
• updated pendulum position URDF to match the expected initial state
• updated CMakeLists to install and register the new tests

---

Mobile robot tests (diff_drive, mecanum, ackermann, tricycle) will be added in a separate PR, since they have no initial_value and require a slightly different test structure.

---

This is an automatic backport of pull request #814 done by Mergify.

[mergify]

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Changes to be committed:
	modified:   gz_ros2_control_demos/launch/gripper_mimic_joint_example_effort.launch.py
	modified:   gz_ros2_control_demos/launch/gripper_mimic_joint_example_position.launch.py
	modified:   gz_ros2_control_demos/launch/pendulum_example_position.launch.py
	modified:   gz_ros2_control_demos/urdf/test_pendulum_position.xacro.urdf
	new file:   gz_ros2_control_tests/tests/gripper_mimic_joint_effort_test.py
	new file:   gz_ros2_control_tests/tests/gripper_mimic_joint_position_test.py
	new file:   gz_ros2_control_tests/tests/pendulum_position_test.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   gz_ros2_control_tests/tests/CMakeLists.txt

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[christophfroehlich]

Waiting for #830