CLN: Split dtype inference tests
Breaking up some very large tests in dtypes/test_inference.py
Hello @dsaxton! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-04-21 02:34:24 UTC small comment request, otherwise LGTM nice cleanup thanks @dsaxton 

would take a followup to use the `nulls_fixture` rather than hard-coding the nan values