[CE] Make spec/lib/gitlab/import_export/all_models.yml EE-ready

What does this MR do?

This ports https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7254 to CE.

This modifies spec/lib/gitlab/import_export/model_configuration_spec.rb to be able to accept an ee/spec/lib/gitlab/import_export/ee_specific_models.yml file for EE-specific model/associations.

The CE version will be the same, but the ee/spec/lib/gitlab/import_export/ee_specific_models.yml file won't be present there.

What are the relevant issue numbers?

Closes gitlab-ee#7471

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides