TST: Add test for mangling of unnamed columns
xref #13017.
Hello @gfyoung! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/io/parser/mangle_dupes.py`](https://github.com/gfyoung/pandas/blob/21d526aa2deeb91dd95a5a15c2d479eab25e415c/pandas/tests/io/parser/mangle_dupes.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23485?src=pr&el=h1) Report
> Merging [#23485](https://codecov.io/gh/pandas-dev/pandas/pull/23485?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0aa9efa5593f87f1268e2dab5b55ee0cc4492458?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/23485/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23485?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23485   +/-   ##
=======================================
  Coverage   92.23%   92.23%           
=======================================
  Files         161      161           
  Lines       51197    51197           
=======================================
  Hits        47220    47220           
  Misses       3977     3977
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `42.27% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23485?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/23485?src=pr&el=footer). Last update [0aa9efa...0834631](https://codecov.io/gh/pandas-dev/pandas/pull/23485?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks.

can we close this issue? Close, but unfortunately, the last part of [this comment](https://github.com/pandas-dev/pandas/issues/13017#issuecomment-215716226) is still present on `master`, hence why I am not closing the issue.