TST: fix period tests for numpy 1.9.3 (GH14183)
Partly addresses #14183 

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14331?src=pr) is 85.26% (diff: 100%)

> Merging [#14331](https://codecov.io/gh/pydata/pandas/pull/14331?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #14331   diff @@
==========================================
  Files           140        140          
  Lines         50630      50630          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43169      43169          
  Misses         7461       7461          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [6dcc238...e39754e](https://codecov.io/gh/pydata/pandas/compare/6dcc23862b6b60ce2a67436b4a278fbe4c05490f...e39754e5cf95d7f65862947b25322c7ff39f0ca7?src=pr)
