Fix unit tests to avoid data race in pipelines

[salonichf5]

Proposed changes

Write a clear and concise description that helps reviewers understand the purpose and impact of your changes. Use the
following format:

Problem: Users want to parallelize unit tests without having data race issues

Solution: Modify unit tests to be independent of variables, maps/structs to avoid data race when tests run in parallel.

Testing: Manual Testing

1. run make unit-test 10-12 times. Passes all the time
2. run tests 10-12 times in the folder https://github.com/nginxinc/nginx-gateway-fabric/tree/main/internal/mode/static/state/graph , where most of the data race failures were happening.

Please focus on (optional): If you any specific areas where you would like reviewers to focus their attention or provide
specific feedback, add them here.

Closes #2569
Closes #2578

Checklist

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.01%. Comparing base (d490d7e) to head (063446f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2602      +/-   ##
==========================================
- Coverage   89.20%   89.01%   -0.19%     
==========================================
  Files         100       99       -1     
  Lines        7642     7413     -229     
  Branches       50        0      -50     
==========================================
- Hits         6817     6599     -218     
+ Misses        768      757      -11     
  Partials       57       57              

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[ADubhlaoich]

PR is primarily code so I will not approve, but the documentation explaining the unit tests is clear.