TYP: Type annotaion for test decorators
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I am not 100% sure this is something we want, or at least with this implementation.

cc @simonjayhawkins 