You can set up your own test instance by going to http://go.atlassian.com/cloud-dev, but this is subject to restrictions (and can be deleted by Atlassian if it goes against the terms of use for a developer instance).
But if it's for the customer to test, then I would say that the trial version is the right way to go. If it's just for a short test to see the functionality, then you may be able to create a temporary account on a test instance (with the caveat that it will be difficult to transfer any information), but I'm not 100% sure how Atlassian feels about that.
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