tests: qemumonitorjson: Test extraction of LUKS node names
    
    Test file created by:
    
    qemu-img create -f luks /var/lib/libvirt/images/luks 10M \
            -o key-secret=asdf --object secret,id=asdf,format=raw,data=asdf
    
    Used in libvirt as:
     <disk type='file' device='disk'>
       <driver name='qemu' type='raw'/>
       <source file='/var/lib/libvirt/images/luks'/>
       <target dev='vda' bus='virtio'/>
       <encryption format='luks'>
         <secret type='passphrase' uuid='9b2c831a-fdb9-4c09-873c-1959580589e1'/>
       </encryption>
     </disk>
    
    Reviewed-by: Eric Blake <eblake@redhat.com>