[backport 2.3.x] TST (string dtype): clean-up assorted xfails (#60345)
(cherry picked from commit e7d1964ab7405d54d919bb289318d01e9eb72cd1)

Backport of #60345
@jorisvandenbossche #59481 is marked as backported but I'm not sure it was so I think that's why you had merge conflicts in pandas/tests/io/excel/test_readers.py and why there is still an xfail on 2.3.x for test_dtype_backend_string 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. > @jorisvandenbossche #59481 is marked as backported but I'm not sure it was so I think that's why you had merge conflicts in pandas/tests/io/excel/test_readers.py and why there is still an xfail on 2.3.x for test_dtype_backend_string

That PR was certainly backported _at least partly_, but of course it might always be that some merge conflicts were resolved incorrectly so that not every change was fully applied to 2.3.x. 

And indeed in test_readers.py there seems to be some leftovers. Thanks for the notice! > And indeed in test_readers.py there seems to be some leftovers. 

Turns out it was only the xfail itself that was not properly removed, the actual test code changes were backported properly -> fixup in https://github.com/pandas-dev/pandas/pull/60599