TST: check inequality by comparing categorical with NaN ( #28384 )
- [x] closes #28384
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
Please review.

And I need some help.  
I don't know why CI is failing.From the looks of it, it looks like it's timed out. > I think possibly add a non-categorical example since the OP was about the difference in behavior between categorical vs. non-categorical. See [#28384 (comment)](https://github.com/pandas-dev/pandas/issues/28384#issuecomment-530514218)
> 
> Re: CI failures those are unrelated I think. Merge with master and they'll be rerun

Thank you for your input. I added test case.
And I rebased my branch. thanks @junjunjunk 