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Without seeing the content of test.launch I cannot be entirely certain. That said, I can say and guess the following:

• The error RLException: Invalid <param> tag: Cannot load command parameter [robot_description] is happening because xacro failed to return an output, as you guessed.
• xacro probably couldn't find the .xacro file, as the error No such file or directory: /home/ylh/catkin_ws/src/arobot_ws/test/urdf/test.urdf.xacro [Errno 2] No such file or directory: indicates.

I'd guess test.urdf.xacro isn't present in the path xacro can look up. I see $(find test) in the launch file. On command line (e.g. on bash), make sure rospack find test returns the absolute path that test should reside in.