Fix flakiness of qos/test_qos_dscp_mapping.py (#13496)

What is the motivation for this PR?
In some of the cases, after sending packets (2000) the queue counter value is not reflecting correct value (counter value is less than expected) and thus lead to test failure. In the issue state, reading the counter value (in breakpoint) again shows the correct value.

Sample output in failure case:

 3238         "  Inner Packet DSCP Value    Expected Egress Queue    Egress Queue Count  Result                              Actual Egress Queue\n",
 3239         "-------------------------  -----------------------  --------------------  --------------------------------  ---------------------\n",
 3240         "                        0                        1                   865  FAILURE - DUT POLL FAILURE                           -1\n",
 3241         "                        1                        1                  2000  SUCCESS                                               1\n",
 3242         "                        2                        1                  1022  FAILURE - DUT POLL FAILURE                           -1\n",
 3243         "                        3                        3                  2000  SUCCESS                                               3\n",
 3244         "                        4                        4                  1396  FAILURE - DUT POLL FAILURE                           -1\n",
 3245         "                        5                        1                  2000  SUCCESS                                               1\n",
 3246         "                        6                        1                  1449  FAILURE - DUT POLL FAILURE                           -1\n",
 3247         "                        7                        1                  2000  SUCCESS                                               1\n",
 3248         "                        8                        0                  1909  FAILURE - INCORRECT PACKET COUNT                      0\n",
 3249         "                        9                        1                  2000  SUCCESS                                               1\n",
...
How did you do it?
Updated the test to wait for atleast 10s (which is hardware counter polling time) before reading the queue counters.
$ counterpoll show | grep -i queue_stat
QUEUE_STAT                  default (10000)     enable
Updated the logic to re-poll the counters if the egress packet count is not as expected.
How did you verify/test it?
Stressed the test with fix on Arista-7260CX3-D108C8. Test is passing consistently with the fix.

tests/qos/test_qos_dscp_mapping.py

@@ -15,7 +15,7 @@
 from tests.common.helpers.ptf_tests_helper import downstream_links, upstream_links, select_random_link,\
     get_stream_ptf_ports, get_dut_pair_port_from_ptf_port, apply_dscp_cfg_setup, apply_dscp_cfg_teardown # noqa F401
 from tests.common.utilities import get_ipv4_loopback_ip, get_dscp_to_queue_value, find_egress_queue,\
-    get_egress_queue_pkt_count_all_prio
+    get_egress_queue_pkt_count_all_prio, wait_until
 from tests.common.helpers.assertions import pytest_assert
 from tests.common.fixtures.duthost_utils import dut_qos_maps_module # noqa F401
 
@@ -127,8 +127,6 @@ def send_and_verify_traffic(ptfadapter,
         logger.info("Received packet(s) on port {}".format(ptf_dst_port_ids[port_index]))
         global packet_egressed_success
         packet_egressed_success = True
-        # Wait for packets to be processed by the DUT
-        time.sleep(8)
         return ptf_dst_port_ids[port_index]
 
     except AssertionError as detail:
@@ -280,15 +278,10 @@ def _run_test(self,
             if packet_egressed_success:
                 dut_egress_port = get_dut_pair_port_from_ptf_port(duthost, tbinfo, dst_ptf_port_id)
                 pytest_assert(dut_egress_port, "No egress port on DUT found for ptf port {}".format(dst_ptf_port_id))
+                # Wait for the queue counters to be populated.
+                verification_success = wait_until(60, 2, 0, lambda: find_queue_count_and_value(duthost,
+                                                  queue_val, dut_egress_port)[0] >= DEFAULT_PKT_COUNT)
                 egress_queue_count, egress_queue_val = find_queue_count_and_value(duthost, queue_val, dut_egress_port)
-                # Re-poll DUT if queue value could not be accurately found
-                if egress_queue_val == -1:
-                    time.sleep(2)
-                    egress_queue_count, egress_queue_val = find_queue_count_and_value(duthost, queue_val,
-                                                                                      dut_egress_port)
-                # Due to protocol packets, egress_queue_count can be greater than expected count.
-                verification_success = egress_queue_count >= DEFAULT_PKT_COUNT
-
                 if verification_success:
                     logger.info("SUCCESS: Received expected number of packets on queue {}".format(queue_val))
                     output_table.append([rotating_dscp, queue_val, egress_queue_count, "SUCCESS", queue_val])