Revert "Move plone.formwidget.namedfile from Dexterity alltests group to plone_app_testing alltests group; Add notes for some groups."

This reverts commit 6d2ebe11cd9784ca0281b907a6692e4b45eb1eae.
Add note why plone.formwidget.namedfile seems to have test isolation problems; Move zope_test group around.
Add more notes about test isolation problems.