TST: use s3_resource fixture consistently
- [x] split off from #23731
- [x] tests modified / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

There are two boto tests that are not using the dedicated `s3_resource` fixture which takes care of all the mocking etc. This PR adapts those tests accordingly, which also allows unified treatment of things like #23754.
