[dhcp_relay] Increase wait time for default route check in dhcp_relay test (sonic-net#15976)

What is the motivation for this PR?
There is flaky failure in this case because default route missing

How did you do it?
Increase wait time for it.
Add log for triage

How did you verify/test it?
Run test

Author: yaqiangz

tests/dhcp_relay/conftest.py

@@ -146,7 +146,7 @@ def dut_dhcp_relay_data(duthosts, rand_one_dut_hostname, ptfhost, tbinfo):
 def validate_dut_routes_exist(duthosts, rand_one_dut_hostname, dut_dhcp_relay_data):
     """Fixture to valid a route to each DHCP server exist
     """
-    py_assert(wait_until(120, 5, 0, check_routes_to_dhcp_server, duthosts[rand_one_dut_hostname],
+    py_assert(wait_until(360, 5, 0, check_routes_to_dhcp_server, duthosts[rand_one_dut_hostname],
                          dut_dhcp_relay_data),
               "Packets relayed to DHCP server should go through default route via upstream neighbor, but now it's" +
               " going through mgmt interface, which means device is in an unhealthy status")

tests/dhcp_relay/dhcp_relay_utils.py

@@ -9,6 +9,10 @@
 def check_routes_to_dhcp_server(duthost, dut_dhcp_relay_data):
     """Validate there is route on DUT to each DHCP server
     """
+    output = duthost.shell("show ip bgp sum", module_ignore_errors=True)
+    logger.info("bgp state: {}".format(output["stdout"]))
+    output = duthost.shell("show int po", module_ignore_errors=True)
+    logger.info("portchannel state: {}".format(output["stdout"]))
     default_gw_ip = dut_dhcp_relay_data[0]['default_gw_ip']
     dhcp_servers = set()
     for dhcp_relay in dut_dhcp_relay_data: