TST: Test unnamed columns with index_col for Excel
Closes #18792.

Hello @gfyoung! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/io/test_excel.py`](https://github.com/gfyoung/pandas/blob/6245b599fda9c233e3522c0c86280aba3199af4b/pandas/tests/io/test_excel.py) !

 failing on py2 > failing on py2

If I made this PR in January, this wouldn't be a problem 😉 .  Debugging right now.

Also, looks like this issue was patched when I fixed #20480. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23874?src=pr&el=h1) Report
> Merging [#23874](https://codecov.io/gh/pandas-dev/pandas/pull/23874?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ca70fe6fc3301fc292bc85957a18b45af0d84418?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/23874/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23874?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23874   +/-   ##
=======================================
  Coverage   92.31%   92.31%           
=======================================
  Files         161      161           
  Lines       51471    51471           
=======================================
  Hits        47515    47515           
  Misses       3956     3956
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <ø> (ø)` | :arrow_up: |
| #single | `42.43% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23874?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/23874?src=pr&el=footer). Last update [ca70fe6...7861850](https://codecov.io/gh/pandas-dev/pandas/pull/23874?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Alright, got everything green now!  PTAL. @jreback: Patched the 2.x bug, and all is green. PTAL. thanks @gfyoung 