TST: Debug flaky plotting test

[TomAugspurger]

Maybe closes #19924. I can't reproduce this locally so who knows.

A while back I went through and added plt.subplots() and explicit ax=ax to a bunch of the plotting tests. I apparently missed this one. It seemed to help, but they're flaky tests so who knows.

Also chanegd idx1.equals(idx2) to tm.assert_frame_equal so we get a better error message when it does fail.

[codecov]

Codecov Report

Merging #19925 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #19925      +/-   ##
==========================================
+ Coverage   91.66%   91.69%   +0.02%     
==========================================
  Files         150      150              
  Lines       48974    48974              
==========================================
+ Hits        44894    44906      +12     
+ Misses       4080     4068      -12

Flag	Coverage Δ
#multiple	90.07% <ø> (+0.02%)	⬆️
#single	41.85% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/plotting/_converter.py	66.95% <0%> (+1.73%)	⬆️

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[TomAugspurger]

It passed, not sure if it helped or not.

Merging later today if there are no objections.