started to fixturize pandas/tests/base
Part of #23877
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

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I didn't apply the fixtures to the rest of `test_ops.py` yet since @jreback voiced some [concerns in this comment](https://github.com/pandas-dev/pandas/pull/30147#discussion_r356587642).