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
No, xblockutils test unconditionally tries loading xml from some folder, while tests here are loaded from json data files. It might make sense to rewrite everything using xml files, but that would take some time. So I would not do that until next time we wnt to modify something here.