Add missing add index rubocop spec

What does this MR do?

The add_index rubocop rule doesn't have an associated spec. This MR adds the missing spec file.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content