move test outside loop in backport_linter factory

[MichaelChirico]

Related: #2338. This is not the primary source of inefficiency but should nevertheless remove some overhead.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3fd1bd6) 99.40% compared to head (d523cb1) 99.40%.

❗ Current head d523cb1 differs from pull request most recent head 81be15e. Consider uploading reports for the commit 81be15e to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2339   +/-   ##
=======================================
  Coverage   99.40%   99.40%           
=======================================
  Files         123      123           
  Lines        5585     5585           
=======================================
  Hits         5552     5552           
  Misses         33       33           

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