Refactor test_simple_imputer.py

As an EvalML dev, I would like `test_simple_imputer.py` to be refactored to use a more centralized test dataframe that either lives in `conftest.py` or `test_simple_imputer.py`. The dataframe will probably have to be separate from `test_imputer.py`'s dataframe given that the SimpleIMputer is not set up to impute mixed data frames.