test(pinia-orm): Add belongsToMany self relation test

[CodeDredd]

🔗 Linked issue

closes #1447

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation or readme)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [x] 👌 Enhancement (improving an existing functionality like performance)
• [ ] ✨ New feature (a non-breaking change that adds functionality)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

It's already working. Just adding a test for this case

📝 Checklist

• [ ] I have linked an issue or discussion.
• [ ] I have updated the documentation accordingly.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.72%. Comparing base (9d25099) to head (7411c93).

:exclamation: Current head 7411c93 differs from pull request most recent head f4e0bef. Consider uploading reports for the commit f4e0bef to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1807 +/- ## ======================================= Coverage 99.72% 99.72% ======================================= Files 87 87 Lines 5895 5895 Branches 500 500 ======================================= Hits 5879 5879 Misses 13 13 Partials 3 3 ```

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