Fix qos/test_qos_sai.py (sonic-net#12334)

* Regression introduced by: sonic-net#10941

Downlink ports don't have BGP neighbors in case of T0, hence the above change is
causing the key error.

The proposed fix is to modify the testPorts dictionary with peer_addr_ipv6 only if
it is defined.

tests/qos/qos_sai_base.py

@@ -781,26 +781,32 @@ def __buildTestPorts(self, request, testPortIds, testPortIps, src_port_ids, dst_
                             if sysMap['port_type'] == sysPorts[port_id]['port_type'] and sport != port_id:
                                 dst_all_sys_port.update({sport: sysMap['system_port']})
 
-        return {
+        testPorts = {
             "dst_port_id": dstPort,
             "dst_port_ip": dst_test_port_ips[dstPort]['peer_addr'],
-            "dst_port_ipv6": dst_test_port_ips[dstPort]['peer_addr_ipv6'],
             "dst_port_vlan": dstVlan,
             "dst_port_2_id": dstPort2,
             "dst_port_2_ip": dst_test_port_ips[dstPort2]['peer_addr'],
-            "dst_port_2_ipv6": dst_test_port_ips[dstPort2]['peer_addr_ipv6'],
             "dst_port_2_vlan": dstVlan2,
             'dst_port_3_id': dstPort3,
             "dst_port_3_ip": dst_test_port_ips[dstPort3]['peer_addr'],
-            "dst_port_3_ipv6": dst_test_port_ips[dstPort3]['peer_addr_ipv6'],
             "dst_port_3_vlan": dstVlan3,
             "src_port_id": srcPort,
             "src_port_ip": src_port_ip["peer_addr"],
-            "src_port_ipv6": src_port_ip["peer_addr_ipv6"],
             "src_port_vlan": srcVlan,
             "dst_sys_ports": dst_all_sys_port
         }
 
+        if 'peer_addr_ipv6' in dst_test_port_ips[dstPort]:
+            testPorts.update({"dst_port_ipv6": dst_test_port_ips[dstPort]['peer_addr_ipv6']})
+        if 'peer_addr_ipv6' in dst_test_port_ips[dstPort2]:
+            testPorts.update({"dst_port_2_ipv6": dst_test_port_ips[dstPort2]['peer_addr_ipv6']})
+        if 'peer_addr_ipv6' in dst_test_port_ips[dstPort3]:
+            testPorts.update({"dst_port_3_ipv6": dst_test_port_ips[dstPort3]['peer_addr_ipv6']})
+        if 'peer_addr_ipv6' in src_port_ip:
+            testPorts.update({"src_port_ipv6": src_port_ip["peer_addr_ipv6"]})
+        return testPorts
+
     def __buildPortSpeeds(self, config_facts):
         port_speeds = collections.defaultdict(list)
         for etp, attr in config_facts['PORT'].items():