To configure an IPv6 test address, you use the link-local address, because link-local addresses are tied to the physical interface. Thus, you do not need a separate IP address in the IPv6 situation. For IPv6, the -failover option has the following syntax:
# ifconfig interface-name inet6 -failover
How to Configure a Multipathing Interface Group With Two Interfaces shows the steps you perform for an IPv6 test address.
When a multipathing group has both IPv4 and IPv6 plumbed on all the group's interfaces, you might not need a separate IPv4 test address. The in.mpathd daemon can probe the interfaces by using an IPv6 link-local address. IPv6 link-local addresses are created when IPv6 is plumbed.
Use failover without the dash to turn on the failover attribute of the address.
The only valid IPv6 test address is the link-local address.