TST: test for expanding window with min_periods GH25857
- [x] closes #25857
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Hi @mroeschke, here's the new test - please let me know if it needs any changes.
@mroeschke should close the OP ? thanks @theMogget  Thanks! Could anyone help at the `hacktoberfest-accepted` label to this please? :)  Thanks @theMogget. added.