TST: Add tests for Categorical.is_dtype_equal against Series

[jschendel]

• closes API/BUG: Categorical.is_dtype_equal doesn't compare to Series #16659
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

Issue is already fixed on 0.22.0 (so really probably fixed in 0.21.x), just adding tests to ensure that there's not a regression. Didn't add a whatsnew entry since this behavior already exists in a released version, but can add one if need be.

[codecov]

Codecov Report

Merging #19045 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #19045      +/-   ##
==========================================
- Coverage   91.57%   91.54%   -0.03%     
==========================================
  Files         150      150              
  Lines       48942    48942              
==========================================
- Hits        44818    44804      -14     
- Misses       4124     4138      +14

Flag	Coverage Δ
#multiple	89.9% <ø> (-0.03%)	⬇️
#single	41.75% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/plotting/_converter.py	65.22% <0%> (-1.74%)	⬇️
pandas/util/testing.py	84.74% <0%> (-0.22%)	⬇️

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[jreback]

thanks @jschendel