Add unit tests for `wp_validate_boolean()`.

Props TobiasBg.
See #30238.
Built from https://develop.svn.wordpress.org/trunk@30206


git-svn-id: http://core.svn.wordpress.org/trunk@30206 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Ignore case when checking string 'false' in `wp_validate_boolean()`.

Props TobiasBg, kitchin.
Fixes #30238.
Built from https://develop.svn.wordpress.org/trunk@30207


git-svn-id: http://core.svn.wordpress.org/trunk@30207 1a063a9b-81f0-0310-95a4-ce76da25c4cd