[ TEST ONLY ] Add tests and idl/python ordering fix to #587

[rkent]

This PR is for testing and demo only.

Tests are added for #587. In addition, a suggested fix is proposed that makes sure that combining a rosidl items (like a /msg folder) do not depend on the order listed in CMakeLists.txt. Specifically, the following is added to ament_python_install_registered_packages.cmake:

  foreach(_dir IN LISTS _PACKAGE_DIRS)
    string(FIND "${_dir}" "rosidl_generator_py" SUBSTRING_INDEX)
    if(NOT SUBSTRING_INDEX EQUAL -1)
      list(REMOVE_ITEM _PACKAGE_DIRS ${_dir})
      list(PREPEND _PACKAGE_DIRS ${_dir})
    endif()
  endforeach()

[rkent]

This was just a test.