use tm.assert_equal instead of parametrizing assert funcs

[jbrockmendel]

No description provided.

[pep8speaks]

Hello @jbrockmendel! Thanks for submitting the PR.

• There are no PEP8 issues in the file pandas/tests/indexes/datetimes/test_tools.py !

• There are no PEP8 issues in the file pandas/tests/scalar/test_nat.py !

• There are no PEP8 issues in the file pandas/tests/tseries/offsets/test_offsets.py !

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

• Complete extra results for this file :

file_to_check.py:2453:-258: W605 invalid escape sequence '('
file_to_check.py:2453:-255: W605 invalid escape sequence ')'

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[codecov]

Codecov Report

Merging #22995 into master will decrease coverage by <.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master   #22995      +/-   ##
==========================================
- Coverage    92.2%   92.19%   -0.01%     
==========================================
  Files         169      169              
  Lines       50822    50826       +4     
==========================================
+ Hits        46858    46861       +3     
- Misses       3964     3965       +1

Flag	Coverage Δ
#multiple	90.61% <75%> (-0.01%)	⬇️
#single	42.35% <0%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/util/testing.py	86.18% <75%> (-0.05%)	⬇️

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