Your username in the system_auth.users table is wrong. For kerberos, you should use fully qualified user names, including the realm part. This is because you may want to have two users with the same name, but different realms.
The cassandra user is special - we didn't want to hardcode any particular realm name so we decided to strip realm from this one particular user.
So your users table should look like this:
name | super
-------------------+-------
test@REALM.COM | False
cassandra | True