Add Tests for AP_DDS

[SantamRC]

List of Suggested Tests to cover AP_DDS Library functions:

• [ ] Test ROS2 Service /ap/arm_motors
• [ ] Test ROS2 Service /ap/mode_switch

[Ryanf55]

Ideally, cover the whole API exposed:

 ros2 node info /ardupilot_dds  
/ardupilot_dds
  Subscribers:
    /ap/cmd_vel: geometry_msgs/msg/TwistStamped
    /ap/joy: sensor_msgs/msg/Joy
    /ap/tf: tf2_msgs/msg/TFMessage
  Publishers:
    /ap/battery/battery0: sensor_msgs/msg/BatteryState
    /ap/clock: rosgraph_msgs/msg/Clock
    /ap/geopose/filtered: geographic_msgs/msg/GeoPoseStamped
    /ap/navsat/navsat0: sensor_msgs/msg/NavSatFix
    /ap/pose/filtered: geometry_msgs/msg/PoseStamped
    /ap/tf_static: tf2_msgs/msg/TFMessage
    /ap/time: builtin_interfaces/msg/Time
    /ap/twist/filtered: geometry_msgs/msg/TwistStamped
  Service Servers:
    /ap/arm_motors: ardupilot_msgs/srv/ArmMotors
    /ap/mode_switch: ardupilot_msgs/srv/ModeSwitch
  Service Clients:

  Action Servers:

  Action Clients:

Since the velocity control is vehicle-specific, there may be different tests for that. Thanks for creating this!