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:
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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