How to upgrade job-dsl files to Jenkins 2 pipelines?
No, there still isn't any common practice for this since this is not very common.

I would recommend you to create a Development Jenkins Environment and run tests against it. Right now you will have to do that from scratch since there are not many open source repositories that could help you.

So, how does this Jenkins Development Environment look like?

For me it is a docker-compose file that you can run and get everything you need. If you can automate the creation of the jobs it will be a plus.