Add support for copying files specified in subfolders of tests folder to the recipe folder

[traversaro]

Fix #75 .

After this PR, if a folder named ros-humble-robot-state-publisher exists in the tests folder, all the contents of the files will be copied in the recipe directory of the recipe of the recipe named ros-humble-robot-state-publisher.

[traversaro]

An example of this was added in RoboStack/ros-humble#320, the tests/ros-humble-robot-state-publisher.yaml content is:

tests:
  # Regression test for https://github.com/RoboStack/ros-humble/issues/274
  - script:
    - launch_test robot_state_publisher_smoke_test_launch.py
    requirements:
      run:
        - ros-humble-launch-testing-ros 
    files:
      recipe:
        - robot_state_publisher_smoke_test_launch.py

while the tests/ros-humble-robot-state-publisher/ folder contains the file robot_state_publisher_smoke_test_launch.py