I don't understand why these failures don't show up on ASF or Policeman Jenkins?
I understand now: ASF and my Jenkins don't set tests.haltonfailure to false (except on ASF Clover jobs), so once a test fails in a module, further modules' tests never get run, and since Nightly tests fail with extreme regularity in Solr core, the contrib tests are never being run. (I can't see Policeman Jenkins Groovy script that sets sysprops scripts/linux-random-java8.groovy - Uwe Schindler do you know about Policeman Jenkins's use of tests.haltonfailure?)
The difference for my Jenkins is how tests are run, via a script that runs 4 Ant processes in parallel, and Solrj and contrib tests are run in a separate Ant process. So Solr core failures don't stop contrib tests from running (though Solrj failures do).
I've started a nightly manual run with tests.haltonfailure=false, and I also added that to my Jenkins scripts. I'm not really sure why it ever makes for Jenkins to run with it set to true? If things go well (i.e. tests.haltonfailure does what I think it does), I'll add it to all the ASF and Policeman Jenkins jobs too.