TST: honor encoding in read_fwf for memory-mapped files
- [x] closes #23254
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry: not needed

`encoding` was already working for memory-mapped files in `read_fwf`.
