CLN: Clean test moments for expanding

[charlesdong1991]

• xref REF: window/test_moments.py #30486 REF: Refactor window/test_moments.py  #30542
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[charlesdong1991]

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!

[jreback]

thanks @charlesdong1991 this is good.