Re-enable content libraries runtime tests [FC-0062]

[pomegranited]

Description

Somewhere in the series of refactorings that this app has undergone, it seems that we removed TestCase from the inheritance tree of the tests in openedx/core/djangoapps/content_libraries/tests/test_runtime.py, effectively skipping the test cases.

This PR re-enables the tests and fixes what's broken since they were disabled.

Supporting information

Builds on/replaces: #35531
Private-ref: FAL-3927

Testing instructions

Check the logs of openedx-1-with-lms and openedx-2-with-cms to confirm that tests are run for openedx/core/djangoapps/content_libraries/tests/test_runtime.py. Some will be skipped for the lms, some skipped for the cms, but all tests should be run by one or the other.

Deadline

ASAP -- want to get into Sumac

[openedx-webhooks]

Thanks for the pull request, @pomegranited!

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[pomegranited]

FYI: There's no create_library_block_child method in the api anymore -- and LearningCoreXBlockRuntime doesn't support child blocks, so I've removed parent-child relationships from these tests.