Test coverage improvements

[e-kolpakov]

Background: This PR contains the LibraryContent XBlock, which allows to display library content in a course.
JIRA ticket: Content Libraries MVP
Discussions: Architecture discussed extensively on the wiki and in meetings, then the revised proposal was presented to the Arch Council on Oct. 21 and given thumbs up.
Dependencies: none
Partner information: 3rd party-hosted open edX instance, for an edX solutions client.

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[e-kolpakov]

Last build already have failed cms-acceptance test, which haven't failed in previous build + last commit contained insigificant whitespace changes to make pylint happy, so it's definitely a flaky test.

Previous build: https://jenkins.testeng.edx.org/job/edx-all-tests-auto-pr/21194/
Current build: https://jenkins.testeng.edx.org/job/edx-all-tests-auto-pr/21197/

So, that CMS.Textbooks probably should be included to flaky tests list.

[bradenmacdonald]

@e-kolpakov

Nit: I would prefer to name this new base class LibraryContentTest since I just had to make the same change in https://github.com/edx/edx-platform/pull/6492/files#diff-6 and some of the new tests don't have much to do with the "container" nature of LibraryContent block.

I would also prefer that in the other new test class below we use the name "LibraryContent" rather than "LibraryContainer" since content is the name of the block that we're testing, and "container" is a different (but related) concept in Studio.

[antoviaque]

@cahrens @chrisndodge Would you mind reviewing this PR? It's the result of @e-kolpakov pass on improving the coverage - it doesn't alter functionality so it should be a straightforward review.

The failing build is due to a single flaky bok-choy test, as noted by Eugny above.

[bradenmacdonald]

Build is passing now :)

[cahrens]

Since these are only test additions/changes, I am comfortable with it only being reviewed by the Solutions team. Thank you for adding additional coverage!

[antoviaque]

@cahrens Great, thank you!

[chrisndodge]

@antoviaque @e-kolpakov I went through this. It's a bit out of my domain expertise and I don't think I have any feedback to this and I don't think anyone else on the Solutions Team would have the background as well. Like @cahrens I don't see risk in merging in additional tests (always a good thing). It'd be fine merge in as-is or if you want another opinion, I'd suggest @dmitchell.

EDIT: Plus I see this is a merge to another feature branch and not master.

[antoviaque]

@chrisndodge Yes, it just adds additional tests so this shouldn't be able to break anything, I'll merge. Thank you for having gone through this!