TST: Add test for setting with loc on DataFrame with one row
- [X] closes #15494
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Issue appears to be have been fixed for a while, since 0.21.x at least, just added a test to ensure there's no regression.
  
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19127?src=pr&el=h1) Report
> Merging [#19127](https://codecov.io/gh/pandas-dev/pandas/pull/19127?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d5194e5692a3327e05e9d0f2544e2512708921f3?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/19127/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19127?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19127   +/-   ##
=======================================
  Coverage   91.51%   91.51%           
=======================================
  Files         148      148           
  Lines       48780    48780           
=======================================
  Hits        44639    44639           
  Misses       4141     4141
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.88% <ø> (ø)` | :arrow_up: |
| #single | `41.59% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19127?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/19127?src=pr&el=footer). Last update [d5194e5...f3a2bbf](https://codecov.io/gh/pandas-dev/pandas/pull/19127?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks. will merge after fixing travis :< thanks @jschendel 