TST: remove usages of np.full_like for numpy<1.8.0 compat
- [x] closes ##14276
- [x] passes `git diff upstream/master | flake8 --diff`

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14277?src=pr) is 85.25% (diff: 100%)

> Merging [#14277](https://codecov.io/gh/pydata/pandas/pull/14277?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #14277   diff @@
==========================================
  Files           140        140          
  Lines         50568      50568          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43113      43113          
  Misses         7455       7455          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [755886c...e098fd7](https://codecov.io/gh/pydata/pandas/compare/755886c9a4406e8f5bcfff9d6bbaf40009f3cad4...e098fd734b6bd8a5d750d6ed950d62fdaade3061?src=pr)
 thanks!
