[CI] test_arm result code is flaky

[christophfroehlich]

I saw this several times now, e.g, here

    [test_position-8] SUCCEEDED result code
    [test_position-8] async_send_goal
Error: ROR] [test_position-8]: process has died [pid 40576, exit code 255, cmd '/home/runner/work/ros2_control_ci/ros2_control_ci/.work/target_ws/install/gz_ros2_control_tests/lib/gz_ros2_control_tests/test_position --ros-args'].
    FAIL
    
    ======================================================================
    FAIL: test_arm (gz_ros2_control_tests.TestFixture.test_arm))
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/home/runner/work/ros2_control_ci/ros2_control_ci/.work/target_ws/src/ros-controls/gz_ros2_control/gz_ros2_control_tests/tests/position_test.py", line 155, in test_arm
        launch_testing.asserts.assertExitCodes(proc_info, process=proc_action,
      File "/opt/ros/jazzy/lib/python3.12/site-packages/launch_testing/asserts/assert_exit_codes.py", line 62, in assertExitCodes
        assert info.returncode in allowable_exit_codes, 'Proc {} exited with code {}'.format(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    AssertionError: Proc test_position-8 exited with code 255
    
    ----------------------------------------------------------------------
    Ran 1 test in 9.157s
    
    FAILED (failures=1)
    [INFO] [python3-5]: sending signal 'SIGINT' to process[python3-5]
    [INFO] [robot_state_publisher-4]: sending signal 'SIGINT' to process[robot_state_publisher-4]
    [INFO] [parameter_bridge-2]: sending signal 'SIGINT' to process[parameter_bridge-2]
    [INFO] [gazebo-1]: sending signal 'SIGINT' to process[gazebo-1]
    [robot_state_publisher-4] [INFO] [1733712495.929158166] [rclcpp]: signal_handler(signum=2)
    [INFO] [python3-5]: process has finished cleanly [pid 40448]
    [parameter_bridge-2] [INFO] [1733712495.931029628] [rclcpp]: signal_handler(signum=2)
    [gazebo-1] Killed
Error: ROR] [gazebo-1]: process has died [pid 40443, exit code -2, cmd 'ruby $(which gz) sim --headless-rendering -s -r -v 4 empty.sdf --force-version 8'].
    [INFO] [robot_state_publisher-4]: process has finished cleanly [pid 40446]
    [INFO] [parameter_bridge-2]: process has finished cleanly [pid 40444]