How to upgrade job-dsl files to Jenkins 2 pipelines? Is there a common practice for migrating a large repository of Jenkins job-dsl to Jenkins2 pipelines? I am looking for ways to test the behaviour of builds, that go a little further than comparison of generated artifact from same code base.

Actually we have a quite large infrastructure, deployed via Ansible, and we got a verification environment which is topologically equivalent to production, so that would not be a problem. I am more interested in validation practices we could automate, as we build more from than 300 project repositories, there are a few thousand jobs spread across a few Jenkins masters.
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.