CLN/TST: Convert `tests/tseries/offset` to pytest idioms
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).


* Converted `setup_method` to `pytest.fixture`s where needed
* Removed `Base` class (tests were moved in #49017) and refactored to a stand alone function

Addressed and green thanks @mroeschke 