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
    container's gateway.
    
    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.