Use embedded workbench to run tests.

Instead of having to checkout the `xblock-sdk` repository
and set `xblock-drag-and-drop-v2` to run within it,
the `xblock-sdk` repository has been added to the test requirements.

A barebone django app that loads the workbench to run integration tests
has been created inside the tests folder.

All test dependencies have been explicitly pinned in `test/requirements.txt`.

Assuming you have Firefox installed, running the tests should be
as simple as `pip install -r tests/requirements.txt` in a fresh virtual
environment and then `tests/manage.py test`.