ERP5TypeTestCase: stop copying ZopeTestCase's publish implementation

Call the original method instead of copy/pasting the code

We still keep the following patches:
 - user: this is an ERP5 addition and maybe we use it in customer project
   code (if we don't I'm in favor of removing the feature, it's used in
   only one place in this repo)
 - call `self.changeSkin(self.portal.getCurrentSkinName())`. Maybe this
   is needed for CMFCore tests and could be good to send upstream, I did
   not check this part