Add a test case to verify that BGP packets use queue7 (sonic-net#8097)

* All control packets including BGP are expected to use Q7, separate
  from data packet queues
* Check for q7 counters is not being done until regression due to
  sonic-net/sonic-swss#2143 is fixed

Signed-off-by: Prabhat Aravind <paravind@microsoft.com>

docs/testplan/BGP-queue-test-plan.md

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+# BGP-queue test plan
+
+* [Overview](#Overview)
+   * [Scope](#Scope)
+   * [Testbed](#Testbed)
+* [Setup configuration](#Setup%20configuration)
+* [Test cases](#Test%20cases)
+
+## Overview
+The purpose is to make sure that BGP control packets use unicast queue 7 by default on all SONiC platforms.
+The test expects that basic BGP configuration for the test is pre-configured on SONiC device before test run.
+
+### Scope
+The test is targeting a running SONiC system with fully functioning configuration. The purpose of the test is to verify BGP control packets use egress queue 7 on SONiC platforms.
+
+### Testbed
+The test could run on any testbed.
+
+## Setup configuration
+This test requires BGP neighbors to be configured and established before the test run.
+
+## Test
+The test will verify that all BGP packets use unicast queue 7 by default. It is to be noted that if BGP sessions are established over PortChannels, LACP packets will also use the same unicast queue 7, but that does not impact the test functionality.
+
+## Test cases
+### Test case test_bgp_queue
+#### Test steps
+* Clear all queue counters using "sonic-clear counters" command
+* Generate a mapping of neighbors to the corresponding interfaces/ports using ARP/NDP entries
+* For all "established" BGP sessions, run "show queue counters" on the corresponding port
+* Verify that unicast queue 7 counters are non-zero and that unicast queue 0 counters are zero

tests/bgp/test_bgp_queue.py

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+import time
+import pytest
+import logging
+
+logger = logging.getLogger(__name__)
+
+pytestmark = [
+    pytest.mark.topology('any'),
+    pytest.mark.device_type('vs')
+]
+
+
+def clear_queue_counters(duthost):
+    duthost.shell("sonic-clear queuecounters")
+
+
+def get_queue_counters(duthost, port, queue):
+    """
+    Return the counter for a given queue in given port
+    """
+    cmd = "show queue counters {}".format(port)
+    output = duthost.shell(cmd)['stdout_lines']
+    txq = "UC{}".format(queue)
+    for line in output:
+        fields = line.split()
+        if fields[1] == txq:
+            return int(fields[2])
+    return -1
+
+
+def test_bgp_queues(duthosts, enum_frontend_dut_hostname, enum_asic_index, tbinfo):
+    duthost = duthosts[enum_frontend_dut_hostname]
+    clear_queue_counters(duthost)
+    time.sleep(10)
+    bgp_facts = duthost.bgp_facts(instance_id=enum_asic_index)['ansible_facts']
+    mg_facts = duthost.get_extended_minigraph_facts(tbinfo)
+
+    arp_dict = {}
+    ndp_dict = {}
+    processed_intfs = set()
+    show_arp = duthost.command('show arp')
+    show_ndp = duthost.command('show ndp')
+    for arp_entry in show_arp['stdout_lines']:
+        items = arp_entry.split()
+        if (len(items) != 4):
+            continue
+        ip = items[0]
+        iface = items[2]
+        arp_dict[ip] = iface
+    for ndp_entry in show_ndp['stdout_lines']:
+        items = ndp_entry.split()
+        if (len(items) != 5):
+            continue
+        ip = items[0]
+        iface = items[2]
+        ndp_dict[ip] = iface
+
+    for k, v in list(bgp_facts['bgp_neighbors'].items()):
+        # Only consider established bgp sessions
+        if v['state'] == 'established':
+            assert (k in arp_dict.keys() or k in ndp_dict.keys())
+            if k in arp_dict:
+                ifname = arp_dict[k].split('.', 1)[0]
+            else:
+                ifname = ndp_dict[k].split('.', 1)[0]
+            if ifname in processed_intfs:
+                continue
+            if (ifname.startswith("PortChannel")):
+                for port in mg_facts['minigraph_portchannels'][ifname]['members']:
+                    logger.info("PortChannel '{}' : port {}".format(ifname, port))
+                    for q in range(0, 7):
+                        assert(get_queue_counters(duthost, port, q) == 0)
+            else:
+                logger.info(ifname)
+                for q in range(0, 7):
+                    assert(get_queue_counters(duthost, ifname, q) == 0)
+            processed_intfs.add(ifname)