Integration tests with custom data

Based off the following PRs:
* https://github.com/edx-solutions/xblock-drag-and-drop-v2/pull/22
* https://github.com/edx/XBlock/pull/244

Adds integration tests against custom data
_get_correct_item_for_zone might reflect the purpose better.
(first I had the impression it moves the correct item into the zone.)