Add spec to verify import test json

What does this MR do?

We want to test Import on "complete" data set, which means that every relation (as in our Import/Export definition) should be covered. Test JSON fixtures, such as spec/fixtures/lib/gitlab/import_export/complex/project.json, expected to include all the relations we list in our Import/Export config.

This MR introduces a spec that will check will fail if new relations will be added into the Import/Export config and will not be included in our Import test fixtures.

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
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• Style guides
• [-] Database guides
• [-] Separation of EE specific content