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
Will the tests run and pass, even if the image is not available?
(While internet connection is generally available, it's better not to depend on them.)