Add missing abstract methods from UserInterface in User test model

This allows to run Laravel-MongoDB tests without the following error:

"PHP Fatal error:  Class User contains 3 abstract methods and must therefore be declared abstract or implement the remaining methods (Illuminate\Auth\UserInterface::getRememberToken, Illuminate\Auth\UserInterface::setRememberToken, Illuminate\Auth\UserInterface::getRememberTokenName) in /.../Laravel-MongoDB/tests/models/User.php on line 92"