Run initial upstream integration test
check-menu is a very robust test that also does not change the host too
much. This does not cover a lot of functionality to actually find
regressions in subsystems, but exposes the missing pieces for
running more integration tests in the dist-git pipeline.
Re-use the cockpit/tests container for this, which has the necessary
test dependencies -- most importantly, chromium-browser. Aside from not
being available in RHEL, this also better shields the tests from the
tested system. The container talks to Cockpit on the host, i. e. the
Use a fixed tag to avoid running into failures with "latest", as we
don't gate container updates on the dist-git tests for now.