Ensure `sanitize_user()` expected test values are lowercase on multisite.

[30524] neglected to account for the fact that multisite forces user logins to
lowercase.

See #30284.
Built from https://develop.svn.wordpress.org/trunk@30527


git-svn-id: http://core.svn.wordpress.org/trunk@30516 1a063a9b-81f0-0310-95a4-ce76da25c4cd