Improves and fixes remaining work_package specs.

Pulls the two recent additions apart into eight separate specs, as well
as fixing the second (i.e. the latter four) specs.
Merge branch 'feature/fix-specs' into feature/merge_planning_element_type_with_type
Merge branch 'feature/merge_planning_element_type_with_type' of github.com:opf/openproject into feature/merge_planning_element_type_with_type