TST: simplify tests for GH14346
Addresses my remaining comment from https://github.com/pandas-dev/pandas/pull/14346#issuecomment-255520873

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14502?src=pr) is 85.26% (diff: 100%)

> Merging [#14502](https://codecov.io/gh/pandas-dev/pandas/pull/14502?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14502   diff @@
==========================================
  Files           140        140          
  Lines         50670      50670          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43205      43206     +1   
+ Misses         7465       7464     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [e3d943d...061484b](https://codecov.io/gh/pandas-dev/pandas/compare/e3d943d1876e8914036e9c323f1876e52da34a04...061484bd3802a967e750aa9ba897ca15d7bee0cd?src=pr)
 lgtm. you _might_ even want to put them in `TestDatetimeLike` (and the index is generic), but might be slightly complicated / not worth it. 
 Yeah, the fact that the test is slightly different (freq should be None or not) will indeed make it more complicated I think
