Fix unknown unicast traffic issue (sonic-net#10884)

What is the motivation for this PR?
test_acl_outer_vlan failed on some testbeds, and root cause is unknown unicast traffic discarded on leaf fanout switch.

How did you do it?
Send packet to leaf fanout switch to learn MAC address, and then leaf fanout can find destination for unicast traffic.

How did you verify/test it?
Run test_acl_outer_vlan test.

tests/acl/test_acl_outer_vlan.py

@@ -546,6 +546,12 @@ def _do_verification(self, ptfadapter, duthost, tbinfo, vlan_setup_info, ip_vers
         if stage == EGRESS:
             # Wait arp
             pytest_assert(wait_until(30, 1, 0, check_arp_status, duthost, dst_ip), "arp table is not updated")
+            # Learn MAC on leaf-fanout to avoid unknown unicast traffic
+            switch_arptable = duthost.switch_arptable()['ansible_facts']
+            mac = switch_arptable['arptable']['v4'][dst_ip]['macaddress']
+            mac_pkt = testutils.simple_tcp_packet(eth_src=mac)
+            for port in dst_port:
+                testutils.send(ptfadapter, port, mac_pkt)
 
         table_name = ACL_TABLE_NAME_TEMPLATE.format(stage, ip_version)
         try: