TST: AmbiguousTimeError with set_index()
- [x] closes #12920
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

this has been fixed by #13750. added tests.

One typo, looks good!
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13814?src=pr) is 85.23% (diff: 100%)

> Merging [#13814](https://codecov.io/gh/pydata/pandas/pull/13814?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13814   diff @@
==========================================
  Files           140        140          
  Lines         50420      50420          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42974      42976     +2   
+ Misses         7446       7444     -2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [31f8e4d...f6696ff](https://codecov.io/gh/pydata/pandas/compare/31f8e4dc8af8f0d109f366d0b726aef210bf7904...f6696ffacdba82b203cd425e1ba6d527ccbd9ed1?src=pr)
 thanks!
