Add tests for attribute whitelist/blacklist for migration
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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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