obsutil: move 'successorssets' to the new modules We have a new 'obsutil' module now. We move this high level utility there to bring 'obsolete.py' back to a more reasonable size.

tests: fix variable name regular expression in _genrestoreenv() Update the code to correctly anchor the expression on the end of the name, to require that the entire name match this expression. It was already anchored at the start by using re.match(), but this does not anchor it at the end.