[autoform] Fix display_fields handling
This follows-up on changeset b2ceb483e056 (present in 3.25 as well), in
which AutomaticEntityForm.editable_attributes() was turned into a
generator. In this method, in case display_fields is not None, we
previously returned a list of (rtype, role); this was changed into a
yield, but the change was missing a return after the loop in order not
to go through the remainder of the function.
Add tests for editable_attributes() method.