Factor meat out of advisory_post and do it in postfail too
    
    If an update test fails before reaching advisory_post, we don't
    generate the 'what update packages were installed' and 'were
    any update packages *not* installed when they should have been'
    logs, but these may well be useful for diagnosing the failure -
    so let's also do the same stuff there. Only let's not do it all
    twice.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>