Prefix the `notoptions` cache key in the multisite `site-options` group with `$wpdb->siteid` to avoid collisions and race conditions when using a fancy multi-network setup. Adds unit test.

Fixes #25883.


Built from https://develop.svn.wordpress.org/trunk@26304


git-svn-id: http://core.svn.wordpress.org/trunk@26209 1a063a9b-81f0-0310-95a4-ce76da25c4cd