Meets #7236: Filtering for assignee's role returns wrong results

[``* `#7236` Filtering for assignee's role returns wrong results``](https://www.openproject.org/work_packages/7236)
This feels wrong: Actually we want to check whether the _result_ contains work packages of ``user_2``. Now you just check if there is a specific statement in the query. What if the statement changes? Please change it in a way that the result of query is tested.

Additionally, try to use ``:member_in_project`` and ``:member_through_role`` of ``user_factory.rb`` to make the spec more concise.