CLN: Clean test moments for expanding
- xref #30486 #30542 
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello, @jreback and @mroeschke 

This is the first follow-up PR for #30542 , and I plan to clean it in three steps in order to keep PR small and readable:

1. this PR will focus on `expanding`
2. the second PR will focus on `ewm`
3. the last PR will focus on `_test_moments_consistency` which is in `common.py` and this will probably influence all three files since they all inherit from this (`expanding`, `ewm` and `rolling`)

If you have any other suggestions on how to proceed, feel free to bring up! thanks @charlesdong1991 this is good.