deflake LearningGoals by switching from useMemo to useEffect

[davidsbailey]

Follows #64282. Speculative fix for https://codedotorg.atlassian.net/browse/TEACH-1639. Done in this PR:

• clean up the afterInit event handler as soon and reliably as possible via useEffect, rather than waiting until test cleanup
• converted the corresponding test case to RTL, because enzyme shallow rendering does not trigger useEffect callbacks

A few reasons why my attention was drawn to this as a potential fix:

• googling the error message suggests a problem with timers or listeners
• LearningGoalsTest uses studioApp while other rubrics tests do not
• useMemo is not the right tool for triggering side effects, even in cases where you do not use a cleanup function

Testing story

• updated unit tests

Deployment strategy

merge #64282 and this PR into staging. wait a few days before merging any additional follow up work.

Future work

potential next steps:

• modernize LearningGoalsTest by switching fully to RTL and Jest.
• experiment with jest --detectOpenHandles (requires a repro, too slow to add to ci)