REF/TST: Add more pytest idiom to util/test_util

[gfyoung]

Also breaks up test_util into multiple test modules by the function or method tested.

[pep8speaks]

Hello @gfyoung! Thanks for submitting the PR.

• There are no PEP8 issues in the file pandas/tests/util/test_deprecate_kwarg.py !

• There are no PEP8 issues in the file pandas/tests/util/test_locale.py !

• There are no PEP8 issues in the file pandas/tests/util/test_move.py !

• There are no PEP8 issues in the file pandas/tests/util/test_safe_import.py !

• There are no PEP8 issues in the file pandas/tests/util/test_util.py !

• There are no PEP8 issues in the file pandas/tests/util/test_validate_args.py !

• There are no PEP8 issues in the file pandas/tests/util/test_validate_args_and_kwargs.py !

• There are no PEP8 issues in the file pandas/tests/util/test_validate_kwargs.py !

[jreback]

looks fine. ping on green.

[codecov]

Codecov Report

Merging #24141 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #24141      +/-   ##
==========================================
+ Coverage    92.2%    92.2%   +<.01%     
==========================================
  Files         162      162              
  Lines       51700    51700              
==========================================
+ Hits        47670    47671       +1     
+ Misses       4030     4029       -1

Flag	Coverage Δ
#multiple	90.6% <ø> (ø)	⬆️
#single	43.02% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/util/testing.py	87.41% <0%> (-0.1%)	⬇️
pandas/io/json/json.py	93.09% <0%> (+0.47%)	⬆️

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[jreback]

thanks @gfyoung