TST: consistent result in dropping NA from CSV
Closes https://github.com/pandas-dev/pandas/issues/21131
<strike>@pandas-dev/pandas-core :

https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=20243

Any ideas as to why this would be failing consistently?  `master` (d5699051) is okay, and this branch is based off `master`, and I don't think this commit is the cause for the issue.</strike>

Being tracked and handled in https://github.com/pandas-dev/pandas/issues/29432 thanks @gfyoung 