Add more 502 checks for test backends

[vepatel]

Proposed changes

• Add more 502 checks in split-traffic tests
• Fix test markers

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• 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

[codecov-commenter]

Codecov Report

Merging #3275 (30ed9d4) into main (2ee3f20) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #3275   +/-   ##
=======================================
  Coverage   52.64%   52.64%           
=======================================
  Files          59       59           
  Lines       16107    16107           
=======================================
  Hits         8480     8480           
  Misses       7343     7343           
  Partials      284      284           

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