DummyMailHost are stored as class attributes rather than instance
attributes.

This behavior broke testPasswordTool and testCRM which instanciate
DummyMailHost and then check for _last_message.



git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@43110 20353a03-c40f-0410-a6d1-a30d3c3de9de