[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.
addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile