The version of Jetty used by the test harness is independent from the version of Jetty used by core, so when core is updated to Jetty 12 EE 8, PCT will still generally be running Jetty 10, which is somewhat unfortunate as it is testing plugins with a different version of Jetty than used in production. While we could go through all critical plugins and update their jenkins.version to Jetty 12 EE 8 and the corresponding Jetty EE 8 test harness, that would be both laborious and aggressive (if Jetty 12 EE 8 is only shipping in a weekly release and hasn't made it to LTS yet).

Perhaps a medium-term compromise can be found as follows. Modify PCT to fetch the version of the test harness in use before starting the test. If the core postdates Jetty 12 EE 8, but the test harness predates the first test release of the test harness with Jetty 12 EE 8, then pass -Djenkins-test-harness.version=<first-release-with-jetty-12-ee8> when invoking PCT. If the core predates Jetty 12 EE 8, don't pass in such an argument (i.e., use whatever version of the test harness we would have otherwise used); similarly, if the plugin has already adopted an even newer test harness, leave it alone rather than downgrading it.

The point of this is to test on a more realistic environment in PCT at runtime, without necessarily forcing plugins to adopt a newer baseline and test harness within their own repositories and Jenkinsfiles.