TST: Fix unused tests in tseries/test_base.py
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- enabled unused patterns left as comments
- made `tz` related tests more comprehensive

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13996?src=pr) is 85.27% (diff: 100%)

> Merging [#13996](https://codecov.io/gh/pydata/pandas/pull/13996?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13996   diff @@
==========================================
  Files           139        139          
  Lines         50206      50215     +9   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42817      42822     +5   
- Misses         7389       7393     +4   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [6f88416...869ef66](https://codecov.io/gh/pydata/pandas/compare/6f8841648d068cc04c7bfe0625f131b7c7d6469e...869ef6698041b439188f8a5f5114c5c7aa5b8795?src=pr)
 thanks!
