test/README: fix advice for testsuite debugging
    
    KERNEL_APPEND="systemd.unit=multi-user.target" is not very useful, because the
    machine will still shut down as soon as the tests succeeds or fails. But
    INTERACTIVE_DEBUG=1 works great, so let's simplify the instructions and
    recommend that.
    
    (cherry picked from commit b13d59243bf0f22c599d9e6be71573e39bf70369)