Вы не сохраняете экземпляры.Вы должны сохранить:
mockDomain(Addresses)
new Addresses(firstLine:'A1', secondLine:'B', thirdLine:'C', luCountry:UnitedStates, zipCode:'12345', luState:Florida, city:'jag').save()
new Addresses(firstLine:'A2', secondLine:'B2', thirdLine:'C2', luCountry:UnitedStates, zipCode:'12344', luState:Florida, city:'jag2').save()
Я бы сделал это так:
mockDomain(Addresses)
mockForContraintsTests(Addresses)
def address1 = new Addresses(firstLine:'A1', secondLine:'B', thirdLine:'C', luCountry:UnitedStates, zipCode:'12345', luState:Florida, city:'jag')
if(address1.validate()) address1.save()
def address2 = new Addresses(firstLine:'A2', secondLine:'B2', thirdLine:'C2', luCountry:UnitedStates, zipCode:'12344', luState:Florida, city:'jag2')
if(address2.validate()) address2.save()
assertEquals 2, Addresses.list().size()