[test] Add regression test for issue #423
[builder] Fix issue #423

Instead of doing the frame change at the append level, do it at the end
in the after_renaming method. In this way, we don't care about the
relative order between translate(), rotate() and append().
[testing] Use properly morse.testing.testing.main
[doc] Fix the documentation related to #423

While here, remove some other outdated comment, like comment about
WheeledRobot and unparent_wheel