TST: tuple and namedtuple multiindex tests for read_csv
- [x] closes #7589
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
#21995 should probably be resolved before this is considered for merging. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21994?src=pr&el=h1) Report
> Merging [#21994](https://codecov.io/gh/pandas-dev/pandas/pull/21994?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1f6ddc4592728c1a09fc0a81ca2a00c685b60a8e?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21994/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21994?src=pr&el=tree)

```diff
@@           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)` | :arrow_up: |
| #single | `42.22% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21994?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21994?src=pr&el=footer). Last update [1f6ddc4...4aea4ed](https://codecov.io/gh/pandas-dev/pandas/pull/21994?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > #21995 should probably be resolved before this is considered for merging. 

is orthogonal no?  @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. @dahlbaek as I said that #21995 is orthogonal.

thanks