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

diff --cc Makefile
Simple merge