Create shared specs for File-based import/export and GitLab 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=385283) </details> <!--IssueSummary end--> ## Summary Create shared specs to make sure that File-based import/export supports same data as GitLab Migration. ## Improvements That will save us from getting some difference between capabilities of both import ways. ## Involved components The current challenge is to provide some shared examples for transformers in GitLab migration and tree restorers for File-based import. As an example here an example of files which should have shared examples between each others. - `spec/lib/bulk_imports/groups/transformers/group_attributes_transformer_spec.rb` - `spec/lib/gitlab/import_export/group/legacy_tree_restorer_spec.rb` **Note:** Bulk import has pretty high number of tranformers for different entities. So, I suspect that those follow-up issue should be promoted into an epic after ~"workflow::refinement"
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