TST: confirm bug in partial string multi-index slicing is fixed (GH12685)
Closes #12685

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13559?src=pr) is **84.34%**

> Merging [#13559](https://codecov.io/gh/pydata/pandas/pull/13559?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13559   diff @@
==========================================
  Files           138        138          
  Lines         51126      51126          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43122      43122          
  Misses         8004       8004          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [f20b41e...3566573](https://codecov.io/gh/pydata/pandas/compare/f20b41e73fdc84089289cd3999762892f85ec4b6...3566573625bda22ad9f5e7ffd1619c70d72263ea)
