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.



git-svn-id: https://develop.svn.wordpress.org/trunk@26304 602fd350-edb4-49c9-b593-d223f7449a82