Store contexts as todos, not tags, step 7: fixing feature tests

Summary:

• port testdata XML to using context items instead of tags
• creating a context now requires a datasource

ctest finally says:
100% tests passed, 0 tests failed out of 99

That doesn't mean the work is done. All it means is that any remaining
bugs will point to holes in the automated tests :-)

Reviewers: ervin

Reviewed By: ervin

Differential Revision: https://phabricator.kde.org/D19146