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 might prefer

    self.assertIn("width: 190px;", items[0].get_attribute('style'))
    self.assertIn("height: auto;", items[0].get_attribute('style'))

As it's more robust against adding new styling parameters, but it's probably fine for now.