TST: split out some sparse tests

[jreback]

No description provided.

[codecov]

Codecov Report

Merging #18968 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #18968      +/-   ##
==========================================
- Coverage   91.57%   91.56%   -0.01%     
==========================================
  Files         150      150              
  Lines       48913    48939      +26     
==========================================
+ Hits        44792    44811      +19     
- Misses       4121     4128       +7

Flag	Coverage Δ
#multiple	89.92% <88.88%> (-0.01%)	⬇️
#single	41.74% <77.77%> (+0.56%)	⬆️

Impacted Files	Coverage Δ
pandas/core/frame.py	97.68% <ø> (ø)	⬆️
pandas/io/clipboards.py	100% <ø> (ø)	⬆️
pandas/core/strings.py	98.47% <ø> (ø)	⬆️
pandas/core/dtypes/dtypes.py	95.27% <100%> (ø)	⬆️
pandas/io/formats/format.py	96.24% <100%> (ø)	⬆️
pandas/io/pytables.py	92.84% <100%> (ø)	⬆️
pandas/io/parsers.py	95.48% <100%> (ø)	⬆️
pandas/core/indexes/interval.py	92.61% <0%> (-1.21%)	⬇️
pandas/core/sparse/array.py	91.82% <0%> (-0.47%)	⬇️
pandas/core/dtypes/cast.py	88.42% <0%> (-0.18%)	⬇️
... and 5 more

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