Atlassian offer free cloud dev environments which you could use for testing like this.
go.atlassian.com/cloud-dev is where you create one.
Some more info here: https://developer.atlassian.com/blog/2016/04/cloud-ecosystem-dev-env/
If you want a full testing environment, you'd have to get a new cloud instance for this purpose.
There's some info from Atlassian here:
You can buy the smallest user tier ($10/month) and keep that as your testing/staging. You'll want to copy your production data over and disable the extra users from production in your testing/staging site. Enable up to 10 users and you can start working in the staging environment.
The following knowledge base article is a good starting place for info about how to copy over a cut of your production data:
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