base tests: run on Cloud, add one on aarch64, clean up priorities
    
    This adds new variant test suites for several of the base tests
    to run them on Cloud disk images. We can't re-use the existing
    test suites because they have `START_AFTER_TEST` defined and
    there's no viable way to undefine it (there are things you can
    do to override the value, but you can't just unset it). Also
    add a stray test we weren't running on aarch64, and correct some
    priorities.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>