add tests for drag and drop unit into collapsed subsection on course out...

...line page
I'm not sure why we need the new OutlineWrapper class. We already have CourseOutlineChild which I think represents the same items by using the three subclasses CourseOutlineSection, CourseOutlineSubsection and CourseOutlineUnit. It certainly has the same name and body selectors, and the same is_browser_on_page implementation.