Rework libregrtest.save_env

* Replace get/restore methods with a Resource class and Resource subclasses
* Create ModuleAttr, ModuleAttrList and ModuleAttrDict helper classes
* Use __subclasses__() to get resource classes instead of using an hardcoded
  list (2 shutil resources were missinged in the list!)
* Don't define MultiprocessingProcessDangling resource if the multiprocessing
  module is missing
* Nicer diff for dictionaries. Useful for the big os.environ dict
* Reorder code to group resources