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

Hello @charlesdong1991! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:









There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-12-30 14:06:20 UTC all of @mroeschke suggestions are good but i’d like to first just move things then actually  break apart - otherwise the diffs are very hard to see @jreback thanks for your comments! Could you please explain a bit more about your point, i think currently this PR is basically simply moving things (except for a couple parametrizations). Or you want them to be split to different files/directories?

Would be nice to hear more so that I could make changes based on @mroeschke nice reviews later on tonight. Thanks!  i want things move with minimal changes in a PR
then you can come back with following PRs to actually parametize / split tests and so on
otherwise the diff is not clear in what is moving / what is actually changing

so pls make this PR solely about moving things to subdirectories (sure you will need some very minor changes to get things to pass) but don’t do anything more than that in this PR Hi, @jreback I revert the change based on your tips, and in this PR i only split tests into three sub-files

To be clear of what i did in this PR:
There is a subfolder now under `window` called `moments`, in which there are three files, for `expanding`, `rolling` and `ewm` separately. And in each file, there are two cases, one for `TestMoments` and one for `TestMomentsConsistency`. 
In order to facilitate it, i made a base class for `MomentsConsistency` tests in `common.py`

@mroeschke many thanks for your nice reviews, following @jreback suggestion, I will address all of them in the follow-up PR and make this PR solely for tests split into sub-files. thanks @charlesdong1991 

yeah happy to have followups to split / clean this.