CLN: refactor tests in test_moments_ewm.py
- [ ] xref: #30486 #30542 
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

thanks @charlesdong1991 
this scope is good. moving common functions out and simplifiying logic & parmeterizing. keep em coming here. thanks, will do @jreback 