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

Built from https://develop.svn.wordpress.org/trunk@30715


git-svn-id: http://core.svn.wordpress.org/trunk@30705 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Customizer: Move private helper functions to `wp.customize.utils` so they can be unit tested.

Includes unit tests.

props ryankienstra, westonruter.
see #28709.
Built from https://develop.svn.wordpress.org/trunk@30716


git-svn-id: http://core.svn.wordpress.org/trunk@30706 1a063a9b-81f0-0310-95a4-ce76da25c4cd