Create shared specs for File-based import/export and GitLab Migration
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## 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"
issue