I agree that no circuit that hosts a Grand Prix should be used for testing. I’d argue that Barcelona might be slightly more interesting if the teams hadn’t already run their cars there a few months previously and already got their setups pretty much tuned in beforehand.
That said, my vote would go for Motorland as well as it looks like a fantastic venue that deserves to at least get a try out.
I think that if they continue to have a “European” Grand Prix then it should run at different circuits every year and give those that don’t already host a GP the chance to prove themselves – that way we could see the likes of Motorland, Imola (the new version), Donington et al have a race without having to commit to a long term contract. Though in reality the European GP should be the first to go if we’re letting other countries onto the calendar.