tests: qemumonitorjson: Test extraction of LUKS node names

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>