Skip dualtor check_nexthops_single_downlink on VS platform (#15470)

Approach
What is the motivation for this PR?
In PR test, check_nexthops_single_downlink have chance to take more than 1 hour because it needs to run several rounds of traffic test, each round would take more than 20 mins, it would easily cause PR test timeout

How did you do it?
Skip earlier in check_nexthops_single_downlink for VS platform, needn't go deep into traffic test

Co-authored-by: xwjiang2021 <[email protected]>

tests/common/dualtor/dual_tor_utils.py

@@ -1184,6 +1184,10 @@ def check_nexthops_single_downlink(rand_selected_dut, ptfadapter, dst_server_add
 
     ptf_t1_intf = random.choice(get_t1_ptf_ports(rand_selected_dut, tbinfo))
     port_packet_count = dict()
+
+    if asic_type == "vs":
+        logging.info("Skipping validation on VS platform")
+        return
     packets_to_send = generate_hashed_packet_to_server(ptfadapter, rand_selected_dut, HASH_KEYS, dst_server_addr,
                                                        expect_packet_num)
 
@@ -1197,10 +1201,6 @@ def check_nexthops_single_downlink(rand_selected_dut, ptfadapter, dst_server_add
         for ptf_idx, pkt_count in ptf_port_count.items():
             port_packet_count[ptf_idx] = port_packet_count.get(ptf_idx, 0) + pkt_count
 
-    if asic_type == "vs":
-        logging.info("Skipping validation on VS platform")
-        return
-
     logging.info("Received packets in ports: {}".format(str(port_packet_count)))
     for downlink_int in expected_downlink_ports:
         # packets should be either 0 or expect_packet_num: