PR for addressing test related notes

Related: https://github.com/edx-solutions/xblock-drag-and-drop-v2/pull/19

Work in progress
I'm not sure what would be the result of this. Do you mean `self._block.data` should be set in `test_*` methods based on `self.initial_data()` dictionary with test-specific modifications made in `test_*` method as well?