test(many-to-many): cover loading HasMany through ManyToMany

[roxblnfk]

🔍 What was changed

• Regression test for loading a HasMany relation of entities loaded through a ManyToMany relation: load('posts')->load('posts.comments'), checked with fetchData() and fetchAll() for JOIN, LEFT_JOIN and POSTLOAD.
• No changes in src/: the bug reported for ORM v1 ("Undefined node") does not reproduce on 2.x, and the test keeps it that way.

Checklist

• Closes Cannot load related entities through many to many relations via LEFT_JOIN #71
• How was this tested:
  • Unit tests added
  • SQLite suite for the new case passes locally (6 tests); MySQL, Postgres and SQL Server are left to CI

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.62%. Comparing base (4dd4ed4) to head (8861148).
⚠️ Report is 1 commits behind head on 2.x.

Additional details and impacted files

@@            Coverage Diff            @@
##                2.x     #577   +/-   ##
=========================================
  Coverage     91.62%   91.62%           
  Complexity     2031     2031           
=========================================
  Files           132      132           
  Lines          5277     5277           
=========================================
  Hits           4835     4835           
  Misses          442      442           

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