[arp_responder] Fix response icmpv6 packet error (sonic-net#9353)

What is the motivation for this PR?
Introduced this PR: sonic-net#8697. Missing specifying interface would causes that ipv6 reply packet is sent by default interface(sendrecv.py), which is incorrect.

How did you do it?
Specify send interface.

How did you verify/test it?
Run this script in ptf and ping ipv6 address from DUT, then run show ndp in DUT, address pinged before can be seen in ndp table.
Run test_acl.py in m0 testbed, all passed. (This test case in m0 would trigger ndp reply)

Signed-off-by: Yaqiang Zhu <[email protected]>

tests/scripts/arp_responder.py

@@ -63,7 +63,7 @@ def reply_to_ndp(data):
 
         ndp_reply = ARPResponder.generate_neigh_adv(ARPResponder.ip_sets[interface][request_ip],
                                                     remote_mac, request_ip, remote_ip)
-        scapy.sendp(ndp_reply)
+        scapy.sendp(ndp_reply, iface=interface)
 
     @staticmethod
     def generate_arp_reply(local_mac, remote_mac, local_ip, remote_ip, vlan_id):