TST: Test DatetimeIndex weekend offset
closes #5348
closes #5115
closes #5152

The original issues pass on master as of 0.19.1. These related issues might have been fixed part of #5004, but it doesn't look like that any PRs fixed this recently.  

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14853?src=pr) is 85.26% (diff: 100%)
> Merging [#14853](https://codecov.io/gh/pandas-dev/pandas/pull/14853?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14853   diff @@
==========================================
  Files           144        144          
  Lines         50981      50981          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43469      43470     +1   
+ Misses         7512       7511     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [b6de920...736ebb2](https://codecov.io/gh/pandas-dev/pandas/compare/b6de920d8c3c2becc46b4fe233e9f388947554f2...736ebb284bf21d13c6c231e8d3f38ad70db748b8?src=pr) @mroeschke Thanks a lot, this is really useful for cleaning up the issues!