Summary:
Making changes to build agent settings in a Jenkinsfile is an important feature available in the editor, but none of our existing tests cover this path. A simple ATH test (or even an addition to another, existing test) would be a nice coverage improvement.

Ideas:
An offline test could create a repository including an existing Jenkinsfile, stored in our resources directory, like we're doing here. This existing Jenkinsfile might include something like:

agent {
    label ("linux")
}

which we could then use the editor to change into

agent any

And then verify that the changes were made correctly.