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)
• 👌 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 ✅

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

❗ 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

@@           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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