move _acceptDivergenceOnInvoice and _adoptDivergenceOnInvoice to TestInvoice class because they are also used in TestPurchaseInvoice

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@31894 20353a03-c40f-0410-a6d1-a30d3c3de9de