accessor holders before deleting the class itself because a reference
on the class will still be kept as bases of erp5.portal_type, thus
this ensures that erp5.portal_type.Foo will be 'unghost' when
accessing constraints and _categories.

Also, move testInstanceBaseCategoryList to TestPortalTypeClass as the
issue is initially specific to portal type class.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@43982 20353a03-c40f-0410-a6d1-a30d3c3de9de