Merge branch 'jc/ci-test-contrib-too'

Test contrib/ things in CI to catch breakages before they enter the "next" branch. * jc/ci-test-contrib-too: : Some of our downstream folks run more tests than we do and catch : breakages in them, namely, where contrib/*/Makefile has "test" target. : Let's make sure we fail upon accepting a new topic that break them in : 'seen'. ci: ubuntu: use GNU coreutils for dirname test: optionally test contrib in CI