Add a basic test case to compare-and-submit

[pmitros]

This adds a basic test case to this XBlock. In order to run the test, use:

paver test_system -t openedx/tests/xblock_integration/test_external_xblocks.py

Note that I have not confirmed whether this works on master. The test framework has several bugs with a PR to fix several bugs in flight (https://github.com/edx/edx-platform/pull/10747), and I'm working off of a branch with those tests fixed.

@strader

[pmitros]

@cahrens FYI: This XBlock may come up for code review in the not-too-distant-future. It was built by Ross. I did code review, and a pair of PRs to bring it up to PEP8, and add a basic test case. I'm not sure what else @strader has in mind for it before he thinks its launch-ready, but on a code review, those were the major gaps I saw with regards to edx.org XBlock standards. I'd wait for feedback from Ross, but I just wanted to get it onto your radar.

[strader]

@pmitros: Thanks for adding this test case. Can you let me know when the test framework is up to date on master? This xblock is already running on the Stanford Open edX instance, so I'd like to wait to merge until we know it's not going to break anything.

[pmitros]

Oh. It's been on master for a while.

[strader]

@pmitros: Great, thanks Piotr. I've merged the test case.