Add a few more tests around multiple choice choice publishing.

Republishing loses student MC Question answers in portal. [#66299458]

https://www.pivotaltracker.com/story/show/66299458
Fixes breaking multiple choice choices when publishing. [#66299458]

The problem was that the key being used for the external_id should have
been a string, but was an integer.

https://www.pivotaltracker.com/story/show/66299458