Use pandas.testing

[jakirkham]

pandas.util.testing is deprecated ( pandas-dev/pandas#30745 ). The preferred way forward is to use pandas.testing. So this updates Dask-cuDF usage accordingly.

[codecov]

Codecov Report

Merging #6357 into branch-0.16 will increase coverage by 0.03%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.16    #6357      +/-   ##
===============================================
+ Coverage        83.18%   83.21%   +0.03%     
===============================================
  Files               92       92              
  Lines            14729    14730       +1     
===============================================
+ Hits             12252    12258       +6     
+ Misses            2477     2472       -5     

Impacted Files	Coverage Δ
python/cudf/cudf/testing/fuzzer.py	0.00% <0.00%> (ø)
python/cudf/cudf/_version.py	45.51% <0.00%> (+0.71%)	⬆️
python/cudf/cudf/core/abc.py	91.48% <0.00%> (+4.25%)	⬆️
python/cudf/cudf/utils/gpu_utils.py	58.53% <0.00%> (+4.87%)	⬆️

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