Tools: ros2: Add missing dep and improved instructions

Signed-off-by: Ryan Friedman <ryanfriedman5410+github@gmail.com>

Tools/ros2/ardupilot_dds_tests/package.xml

@@ -31,6 +31,7 @@
   <exec_depend>geographic_msgs</exec_depend>
   <test_depend>launch</test_depend>
   <test_depend>launch_pytest</test_depend>
+  <test_depend>micro_ros_agent</test_depend>
   <test_depend>launch_ros</test_depend>
   <exec_depend>micro_ros_msgs</exec_depend>
   <test_depend>python3-pytest</test_depend>