[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.

Flake8 cubicweb/web/views/autoform.py Fixes: cubicweb/web/views/autoform.py:703: [E122] continuation line missing indentation or outdented cubicweb/web/views/autoform.py:704: [E122] continuation line missing indentation or outdented cubicweb/web/views/autoform.py:819: [W504] line break after binary operator cubicweb/web/views/autoform.py:820: [E125] continuation line with same indent as next logical line cubicweb/web/views/autoform.py:961: [W504] line break after binary operator cubicweb/web/views/autoform.py:962: [E126] continuation line over-indented for hanging indent