Thanks to info unearthed in this pull request, it should be possible to create a mock pull request for testing the PR tab in blue ocean:

https://github.com/jenkinsci/blueocean-plugin/pull/650

@michaelneale https://github.com/jenkinsci/branch-api-plugin/blob/12b8364d0fb1aa057b5a4ca81317b150a16121e1/src/test/java/integration/CategorizationTest.java#L147-L178 shows creating change requests

@michaelneale if you use "execute groovy script" you can create a controller and set up the controller's repo as you want. then use that from the ath