REF/TST: Add pytest idiom to test_testing

[gfyoung]

Splits test file into multiple new files and delegates remaining tests to the miscellaneous test_util.py

[pep8speaks]

Hello @gfyoung! Thanks for submitting the PR.

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

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

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

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

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

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

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

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

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

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

[codecov]

Codecov Report

Merging #24369 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #24369      +/-   ##
==========================================
+ Coverage   92.29%   92.29%   +<.01%     
==========================================
  Files         162      162              
  Lines       51832    51832              
==========================================
+ Hits        47839    47840       +1     
+ Misses       3993     3992       -1

Flag	Coverage Δ
#multiple	90.7% <ø> (ø)	⬆️
#single	42.99% <ø> (+0.01%)	⬆️

Impacted Files	Coverage Δ
pandas/util/testing.py	87.75% <0%> (+0.09%)	⬆️

---

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[codecov]

Codecov Report

Merging #24369 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #24369      +/-   ##
==========================================
+ Coverage   92.29%   92.29%   +<.01%     
==========================================
  Files         162      162              
  Lines       51841    51841              
==========================================
+ Hits        47847    47849       +2     
+ Misses       3994     3992       -2

Flag	Coverage Δ
#multiple	90.7% <ø> (ø)	⬆️
#single	42.98% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/util/testing.py	87.75% <0%> (+0.18%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5d134ec...aca6404. Read the comment docs.

[jreback]

thanks @gfyoung