TST: Fix test_coercion for period dtype
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`

fix test to meet period dtype #13941.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14025?src=pr) is 85.30% (diff: 100%)

> No coverage report found for **master** at 6fa2b03.
> 
> Powered by [Codecov](https://codecov.io?src=pr). Last update [6fa2b03...b873683](https://codecov.io/gh/pydata/pandas/compare/6fa2b03d78a8a6e94de66a01852a01a5574af2a9...b873683797b8346b1c747a6ab8866580748d97f0?src=pr)
 ty sir!
