[entity] Add a cw_related_rqlst method returning the RQL select node of cw_related method This is useful if one wants to modify the query before execution. A bit a PEP8 style fix in tests along the way. Related to #12306543.

missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/*/cubicweb/skeleton/debian/rules.tmpl)