Update mapping tests for misc. extensions

[mbabker]

• Refactors the reference integrity mapping test to use an annotated document instead of YAML mapping (which is long unsupported in the MongoDB ODM now)
• Adds references mapping tests (only covering annotation/attribute) with both an ORM mapped entity and an ODM mapped document to try and cover both sides of those references
• Refactors the tree mapping test to use XML fixtures instead of YAML
• Removes some duplicate XML specific test cases since this should be covered by the general mapping tests (or cases added for both annotation/attribute and XML)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.33%. Comparing base (3ca38e8) to head (a95cccf).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2951      +/-   ##
==========================================
- Coverage   78.49%   77.33%   -1.17%     
==========================================
  Files         168      168              
  Lines        8813     8818       +5     
==========================================
- Hits         6918     6819      -99     
- Misses       1895     1999     +104     

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