tests: net: iface: verify multicast address not removed if used

If two IPv6 addresses have the same solicited-node multicast address
because they derive from the same EUI-64 interface identifier, make sure
that the solicited-node multicast address is not removed when one of the
IPV6 address is removed but only when both addresses are removed.

Signed-off-by: Florian Vaussard <florian.vaussard@gmail.com>