CLN: Remove unnecessary uses of pd. in tests
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

- Mostly edits along the lines of `pd.Series` -> `Series` for things that were already imported and had redundant uses of `pd`.  
- I cleaned some of the `import` statements, but I didn't import anything new, though that could be done in the future for further cleaning. 
- Was able to shorten some multi-line statements for a net deletion of ~100 lines without changing the content of any tests.
- Only did the base level `tests` and `tests/indexing` so far. 
