Fix logic for _run_test in frames_test.py

[caneff]

Came across this while trying to debug other things, but if a series has a non-unique index run_test would fail because ser.columns doesn't exist.

I guess none of our existing tests have non-unique indices for series.

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[caneff]

R: @tvalentyn

[codecov]

Codecov Report

Merging #28557 (5d6bb03) into master (603b517) will decrease coverage by 0.01%.
Report is 153 commits behind head on master.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #28557      +/-   ##
==========================================
- Coverage   72.34%   72.34%   -0.01%     
==========================================
  Files         682      683       +1     
  Lines      100536   100819     +283     
==========================================
+ Hits        72737    72940     +203     
- Misses      26221    26301      +80     
  Partials     1578     1578              

Flag	Coverage Δ
python	82.82% <ø> (-0.05%)	⬇️

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[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[tvalentyn]

something is not right with test infra

[tvalentyn]

asked in #28539

[tvalentyn]

Checked: Dataframe precommit, formatter and lint GHA tasks, noted BUILD successful in the logs, failure is during teardown and unrelated to the PR.