Unit tests (as in, tests for unit folders)

[rtibbles]

Summary

Adds a test to make sure that units have their assessment items published as a Perseus zip (not QTI, as for now that only happens for Surveys) but skips assessment metadata
Updates publishing logic with helper function has_assessments to handle the two cases consistently in all places we check.
Fixes issue that arose when adding more test data to the export channel test setup - previously the Bad Mastery exercise node did not appear in the complete nodes for some reason, but now it does. Explicitly skips it.
Updates le-utils for updated JSONSchema for mastery criteria.
Adds a requirement that if a folder is a UNIT, it must have valid pre-post-test completion criteria to be considered complete.

Note: This relies on ricecooker code pushing the correct assessment_item_ids at time of push, rather than dynamically generating them.

References

In support of learningequality/kolibri-ecosystem#38

Reviewer guidance

Are there any places where exercise handling has been updated that it shouldn't? Are there any places where exercise handling hasn't been updated but it should?

I have tested this locally, uploading an example course channel to my development server and ensuring the published channel db look as I would expect, with the correct options data on the UNITs, and appropriate file objects in the DB.

Attached here: a0271e8992f856a99a2d1b8ecfde9c5a.zip

Can also be tested more fully with this ricecooker draft PR: learningequality/ricecooker#653

[bjester]

It seems to me ContentNode.mark_complete should be updated. While the validation function was updated here, that method side steps it for topics, meaning a node with Unit modality can be marked complete without necessary mastery criteria. Unless updating that would be problematic?

[bjester]

When publishing the channel from your ricecooker example, I noticed a bunch of error messages about Unable to sort answers, leaving unsorted, although I don't see where that's coming from. Publishing didn't fail though, so I presume it's not an issue.