TST: Add test case when the key is a reserved key
- [ ] closes #24192 
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I saw the reserved_key array on top of the `pandas/core/config.py` file.
```
_reserved_keys = ['all']  # keys which have a special meaning
```

So, I added the test case for test_register_option when the key is a reserved word, which is 'all'.

Please let me know if there is any issue. Thanks.
