TST: read binary file objects with read_fwf
- [x] closes #18035
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry: not needed

`read_fwf` has already supported reading binary file objects. Added the test case from the issue.

thanks @twoertwein 