test: move some library event tests from test_api to test_events [FC-0117]

[bradenmacdonald]

Description

To make my other PR openedx/openedx-core#543 easier to review, and easier to compare its before-and-after effects, this PR moves some content libraries tests from test_api.py to test_events.py, updates them to match the style and detail of the other events tests, and makes them more comprehensive by adding the CONTENT_OBJECT_ASSOCIATIONS_CHANGED event to the event tests, which were previously ignoring it.

This addresses the longstanding TODO in the code:

openedx-platform/openedx/core/djangoapps/content_libraries/tests/test_events.py

Line 595 in eb85705

# TODO: move more of the event-related collection tests from test_api.py to here, and convert them to use REST APIs

Testing instructions

Just make sure the tests are passing and the new tests are similar in coverage and effect to the deleted tests. This PR only changes test code and has no effects on the application code itself.

Deadline

ASAP

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[bradenmacdonald]

@ormsbee @kdmccormick Could I please get a review from one of you on this PR? It is just refactoring the content libraries event tests, without any changes to how the events work.

This change will make it much easier to validate my other PRs that majorly refactor how the library events get emitted.