Make flaky CME test more deterministic

[jonathanmos]

What does this PR do?

Fixes a flaky test that verifies that flattenNode creates a snapshot of the wireframes list before processing, preventing ConcurrentModificationException if the source list is modified during iteration. The test is now deterministic - removing toList from NodeFlattener will always cause the test to fail, while maintaining it will mean the test always passes.

Motivation

Reduce flakiness.

Additional Notes

Anything else we should know when reviewing?

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[datadog-datadog-prod-us1]

🎯 Code Coverage
• Patch Coverage: 100.00%
• Overall Coverage: 65.86% (-0.00%)

View detailed report

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.81%. Comparing base (41c57eb) to head (f8dcf57).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3120      +/-   ##
===========================================
+ Coverage    70.75%   70.81%   +0.07%     
===========================================
  Files          893      893              
  Lines        32983    33000      +17     
  Branches      5547     5549       +2     
===========================================
+ Hits         23334    23368      +34     
- Misses        8089     8091       +2     
+ Partials      1560     1541      -19     

see 37 files with indirect coverage changes

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