TST: split tests/format/test_format.py

[jreback]

closes #15531

[codecov-io]

Codecov Report

Merging #15546 into master will decrease coverage by -0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15546      +/-   ##
==========================================
- Coverage   91.06%   91.04%   -0.03%     
==========================================
  Files         136      136              
  Lines       49104    49104              
==========================================
- Hits        44718    44706      -12     
- Misses       4386     4398      +12

Impacted Files	Coverage Δ
pandas/io/gbq.py	40% <0%> (-46.67%)	❌
pandas/util/decorators.py	67.92% <0%> (-0.95%)	❌
pandas/tools/merge.py	91.78% <0%> (-0.35%)	❌
pandas/core/frame.py	97.74% <0%> (-0.1%)	❌

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