Split get_courses_for_wiki implementation [LMS-2914]

- Move Mongo tests to mixed ddt tests
- Check if `fields` is None before updating search targets
- Fix for double course version
get_courses_for_wiki return course_keys across all modulestores [LMS-2914]
Merge pull request #4431 from edx/split/get_wiki_courses

Split get_courses_for_wiki impl