Add tests for attribute whitelist/blacklist for migration
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=345994) </details> <!--IssueSummary end--> The following discussion from !73754 should be addressed: - [ ] @Alexand started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/73754#note_733875367): (+4 comments) > **question** > > Should we add expectations for the other attrs? Or maybe we already test notes being imported as a sub-relation somewhere else on a unit test? Whatever we decide here we can also extend the question to the other sub-relations on the other context.
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