TST: add timedelta describe
- [x] closes #6145
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`

Must be fixed by `timedelta` dtype. Added tests.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13769?src=pr) is 84.57% (diff: 100%)

> No coverage report found for **master** at b60e42b.
> 
> Powered by [Codecov](https://codecov.io?src=pr). Last update [b60e42b...3348f39](https://codecov.io/gh/pydata/pandas/compare/b60e42b02040624f319cadd60a99fe50eb62d62e...3348f39c6c78b2595aac262cba71ea50257c4e08?src=pr)
 thanks @sinhrks 
