TST: Check ndarray dtype by default
- [x] split from #13080
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`

Also did little cleanup of `testing.pyx` and `testing.py`.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13088?src=pr) is **84.14%**

> Merging [#13088](https://codecov.io/gh/pydata/pandas/pull/13088?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **+<.01%**

``` diff
@@             master     #13088   diff @@
==========================================
  Files           137        137          
  Lines         50261      50238    -23   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          42288      42270    -18   
+ Misses         7973       7968     -5   
  Partials          0          0          
```
1. 2 files (not in diff) in `pandas/tseries` were modified. [more](https://codecov.io/gh/pydata/pandas/commit/a210f799eb54e628e8844c5c5cc1e9d6d865b538/changes?src=pr#70616E6461732F74736572696573) 
   - Misses `-4` 
   - Hits `-22`
2. 2 files (not in diff) in `pandas/io` were modified. [more](https://codecov.io/gh/pydata/pandas/commit/a210f799eb54e628e8844c5c5cc1e9d6d865b538/changes?src=pr#70616E6461732F696F) 
   - Misses `-3` 
   - Hits `-3`
3. File `...ndas/util/testing.py` was modified. [more](https://codecov.io/gh/pydata/pandas/commit/a210f799eb54e628e8844c5c5cc1e9d6d865b538/changes?src=pr#70616E6461732F7574696C2F74657374696E672E7079) 
   - Misses `0` 
   - Partials `0` 
   - Hits `-2`

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [1296ab3...a210f79](https://codecov.io/gh/pydata/pandas/compare/1296ab39d32acaf2c77ed0c185fafe4ebcfedcb3...a210f799eb54e628e8844c5c5cc1e9d6d865b538)
 failing here: https://travis-ci.org/pydata/pandas/jobs/127978632
 thanks @sinhrks 
