Add concurrent editing test cases in Tree

[sejongk]

What this PR does / why we need it?

This PR enhances test coverage for concurrent editing in Tree CRDT.
The 7 skipped tests correspond to the TODOs within the Tree implementation.

Any background context you want to provide?

What are the relevant tickets?

Fixes #

Checklist

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7368454) 81.75% compared to head (4994f05) 81.80%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #710      +/-   ##
==========================================
+ Coverage   81.75%   81.80%   +0.04%     
==========================================
  Files          57       57              
  Lines        4161     4161              
  Branches      805      805              
==========================================
+ Hits         3402     3404       +2     
+ Misses        502      501       -1     
+ Partials      257      256       -1     

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[hackerwins]

Thanks for your contribution.