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>