[backport 2.3.x] TST (string dtype): resolve xfails for frame fillna and replace tests + fix bug in replace for string (#60295)
(cherry picked from commit fae3e8034faf66eb8ef00bcbed73d48e4ef791d3)

Backport of https://github.com/pandas-dev/pandas/pull/60295
This has some failures for which it might be good to first finish https://github.com/pandas-dev/pandas/pull/60183 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.