Added new argument to Replace the MailHost by DummyMailHost. This prevent the instance send emails. 

Also introduced one easy way to replace MailHost by DummyMailHost: ERP5TypeTestCase._setUpDummyMailHost.



git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@33770 20353a03-c40f-0410-a6d1-a30d3c3de9de