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
I know `xblock-utils` wasn't updated by the time this PR was posted, but can you use the SeleniumBaseTest from xblockutils.base_test ?