Fix SAWarning in tests
    
    Fix following warnings in tests caused by the duplicated compose id
    
    SAWarning: New instance <Compose at 0x7f18889b6320> with identity key
    (<class 'cts.models.Compose'>, ('Fedora-Rawhide-20200517.n.1',), None)
    conflicts with persistent instance <Compose at 0x7f18887f1030>
    
    Signed-off-by: Haibo Lin <hlin@redhat.com>