TST: reduce amount of nesting in tests, specifically move core routines higher

[jreback]

No description provided.

[codecov]

Codecov Report

Merging #16030 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #16030      +/-   ##
==========================================
- Coverage      91%   90.99%   -0.01%     
==========================================
  Files         153      153              
  Lines       50469    50469              
==========================================
- Hits        45927    45924       -3     
- Misses       4542     4545       +3

Flag	Coverage Δ
#multiple	88.76% <ø> (-0.01%)	⬇️
#single	40.43% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/common.py	90.68% <0%> (-0.35%)	⬇️
pandas/util/testing.py	80.55% <0%> (-0.19%)	⬇️

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