The object tmnxOamAncpTestTargetId contains the ID of the subscriber
for which the ANCP loopback test must be performed. The content of the
field is interpreted as being a subscriber-id of max 32 chars
(in case the object tmnxOamAncpTestTarget is set to 'subscriberId'),
or as being an acnp-string of max 63 chars (in case the object
tmnxOamAncpTestTarget is set to 'ancpString').