TYP: annotation of __init__ return type (PEP 484) (pandas/tests)
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

According to [documentation](https://pandas.pydata.org/docs/dev/development/contributing_codebase.html#type-hints) pandas uses [PEP484](https://www.python.org/dev/peps/pep-0484/) for type hints.
PEP484 states that `__init__` function should have `-> None` return type annotation.
In this PR I added return type annotation to `__init__` functions in `pandas/tests` module

thanks @EkaterinaKuzkina 