TST: tuple and namedtuple multiindex tests for read_csv

[dahlbaek]

• closes read_csv in Pandas 0.14 loads NaNs when namedtuple is used for column names #7589
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff

[dahlbaek]

#21995 should probably be resolved before this is considered for merging.

[codecov]

Codecov Report

Merging #21994 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #21994   +/-   ##
=======================================
  Coverage   91.96%   91.96%           
=======================================
  Files         166      166           
  Lines       50334    50334           
=======================================
  Hits        46292    46292           
  Misses       4042     4042

Flag	Coverage Δ
#multiple	90.36% <ø> (ø)	⬆️
#single	42.22% <ø> (ø)	⬆️

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[jreback]

#21995 should probably be resolved before this is considered for merging.

is orthogonal no?

[dahlbaek]

@jreback wrote:

is orthogonal no?

Possibly you're right. In any case, I think this PR does what it is supposed to, and #21995 seems likely to be resolved by extending the to_csv/read_csv docs rather than changing any behaviour. Please let me know if you think I missed something obvious in this PR.

[jreback]

@dahlbaek as I said that #21995 is orthogonal.

thanks