Update for newer format of bodhi.update.status.testing message
    
    In https://github.com/fedora-infra/bodhi/pull/5538 we dropped
    most of the `artifact` dict from these messages, things that are
    duplicated with the `update` dict or not needed. I didn't notice
    that this project uses one of the removed keys, so it broke.
    This uses an appropriate replacement from the `update` dict,
    which has been present in these messages since
    https://github.com/fedora-infra/bodhi/pull/4381 in 2022.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>