Test OC_User_Database in Test_User_Database instead of OC_User_Dummy.
Test_User_Backend::getUser() does not return an array, it returns a string.
Use parent:: in Test_User_Database::getUser().
Make OC_User_Dummy::checkPassword() compatible with OC_User_Example.

The user id has to be returned.
Adjust return value tests for checkPassword() to what OC_User_Example says.