Summary:
Editing of preexisting declarative Jenkinsfiles is an important use case for the Blue Ocean Pipeline Editor, and we should create an ATH case which walks through this flow. This test could be written in any number of ways, and used to exercise any number of features currently lacking coverage.

Ideas:
An offline test could create a repository including an existing Jenkinsfile, stored in our resources directory, like we're doing here. We could then use the editor to make changes to that Jenkinsfile and assert that the changes have been made correctly.