testing case sort_index with pd.set_option('use_inf_as_na', True)
- [ ] closes #29687
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Create test cases for sort_index works with 'pd.set_option('mode.use_inf_as_na',True)

Hello @weikhor! Thanks for opening this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:

* In the file [`pandas/tests/frame/methods/test_sort_index.py`](https://github.com/pandas-dev/pandas/blob/e1c5a881d12275ea55c90a46a6d2a4328de9bf0d/pandas/tests/frame/methods/test_sort_index.py):

> [Line 860:1](https://github.com/pandas-dev/pandas/blob/e1c5a881d12275ea55c90a46a6d2a4328de9bf0d/pandas/tests/frame/methods/test_sort_index.py#L860): [W293](https://duckduckgo.com/?q=pep8%20W293) blank line contains whitespace
> [Line 866:1](https://github.com/pandas-dev/pandas/blob/e1c5a881d12275ea55c90a46a6d2a4328de9bf0d/pandas/tests/frame/methods/test_sort_index.py#L866): [W293](https://duckduckgo.com/?q=pep8%20W293) blank line contains whitespace
> [Line 879:1](https://github.com/pandas-dev/pandas/blob/e1c5a881d12275ea55c90a46a6d2a4328de9bf0d/pandas/tests/frame/methods/test_sort_index.py#L879): [W391](https://duckduckgo.com/?q=pep8%20W391) blank line at end of file

 @weikhor can you merge master and update to comments This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. closing as stale. if you want to continue, pls ping and can re-open.