Buildbots don't run tests with -vv and so only log "xxx was modified
by test_xxx" which is not enough to debug such random issue. In many
cases, I'm unable to reproduce the warning and so unable to fix it.
Always logging the value before and value after should help to debug
such warning on buildbots.
* 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