only be enabled in unit tests.
It uses pickle to check if the object is persistent or not, I'm too lazy to do
this by iterating in gc.get_referents graphs.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@17504 20353a03-c40f-0410-a6d1-a30d3c3de9de