TST (string dtype): clean-up interpolate tests
Small follow-up on https://github.com/pandas-dev/pandas/pull/60637: move the code checking for the error for non-numeric data to a separate test, so we can actually still test the "basic" behaviour properly in the case there are only numeric columns. And remove the xfail for the non-range index case.


Milestone should be 2.3 right? 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.