Split and improve multisite tests for upload quota

Break a single test with many assertions into many tests with single assertions.

In the process, provide separate and comprehensive tests for `upload_is_user_over_quota()`, `is_upload_space_available()`, and `get_space_allowed()`.

Also removes a check for `BLOGSUPLOADDIR`, a constant that never existed. New tests will need to be introduced to handle the ms-files group.

See #30080


git-svn-id: https://develop.svn.wordpress.org/trunk@30715 602fd350-edb4-49c9-b593-d223f7449a82
Customizer: Move private helper functions to `wp.customize.utils` so they can be unit tested.

Includes unit tests.

props ryankienstra, westonruter.
see #28709.

git-svn-id: https://develop.svn.wordpress.org/trunk@30716 602fd350-edb4-49c9-b593-d223f7449a82