Fix new string dtype tests for frame folder
This is the first of many folders...
This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. cc @mroeschke

gentle ping Thanks @phofl 