[testgap] Add a test case to verify that control packets are using TX queue 7

[prabhataravind]

Description
On 8102 and 7050-qx-32 platforms, it was observed that control packets (BGP, LACP) share the same TX queue 0 with data packets as opposed to queue 7 potentially causing loss of control packets when link utilization goes 100%. This issue is being addressed to prevent critical events like BGP flaps in production. A sonic-mgmt test also should be added to test this scenario on different platforms.

admin@str2-8102-02:~$ show queue counters Ethernet28
Port TxQ Counter/pkts Counter/bytes Drop/pkts Drop/bytes

---

Ethernet28 UC0 4728 1231118 0 0
Ethernet28 UC1 0 0 0 0
Ethernet28 UC2 0 0 0 0
Ethernet28 UC3 0 0 0 0
Ethernet28 UC4 0 0 0 0
Ethernet28 UC5 0 0 0 0
Ethernet28 UC6 0 0 0 0
Ethernet28 UC7 0 0 0 0
Ethernet28 MC8 0 0 0 0
Ethernet28 MC9 0 0 0 0
Ethernet28 MC10 0 0 0 0
Ethernet28 MC11 0 0 0 0
Ethernet28 MC12 0 0 0 0
Ethernet28 MC13 0 0 0 0
Ethernet28 MC14 0 0 0 0
Ethernet28 MC15 0 0 0 0

admin@str2-8102-02:~$ ping 20.20.28.20 -c 10
PING 20.20.28.20 (20.20.28.20) 56(84) bytes of data.
64 bytes from 20.20.28.20: icmp_seq=1 ttl=64 time=0.944 ms
64 bytes from 20.20.28.20: icmp_seq=2 ttl=64 time=0.839 ms
64 bytes from 20.20.28.20: icmp_seq=3 ttl=64 time=0.219 ms
64 bytes from 20.20.28.20: icmp_seq=4 ttl=64 time=0.389 ms
64 bytes from 20.20.28.20: icmp_seq=5 ttl=64 time=1.07 ms
64 bytes from 20.20.28.20: icmp_seq=6 ttl=64 time=1.01 ms
64 bytes from 20.20.28.20: icmp_seq=7 ttl=64 time=0.444 ms
64 bytes from 20.20.28.20: icmp_seq=8 ttl=64 time=0.353 ms
64 bytes from 20.20.28.20: icmp_seq=9 ttl=64 time=0.662 ms
64 bytes from 20.20.28.20: icmp_seq=10 ttl=64 time=0.855 ms

admin@str2-8102-02:~$ show queue counters Ethernet28
Port TxQ Counter/pkts Counter/bytes Drop/pkts Drop/bytes

---

Ethernet28 UC0 4740 1232848 0 0
Ethernet28 UC1 0 0 0 0
Ethernet28 UC2 0 0 0 0
Ethernet28 UC3 0 0 0 0
Ethernet28 UC4 0 0 0 0
Ethernet28 UC5 0 0 0 0
Ethernet28 UC6 0 0 0 0
Ethernet28 UC7 0 0 0 0
Ethernet28 MC8 0 0 0 0
Ethernet28 MC9 0 0 0 0
Ethernet28 MC10 0 0 0 0
Ethernet28 MC11 0 0 0 0
Ethernet28 MC12 0 0 0 0
Ethernet28 MC13 0 0 0 0
Ethernet28 MC14 0 0 0 0
Ethernet28 MC15 0 0 0 0

admin@str2-8102-02:~$ show arp
Address MacAddress Iface Vlan

Steps to reproduce the issue:

1. Load a sonic image from any one of master/201911/202012 branch
2. Ping a neighbor on any of the UP interfaces
3. Run "show queue counters " to see which TX queue count keeps increasing

Describe the results you received:
UC Q0 count keeps increasing with each icmp packet.

Describe the results you expected:
UC Q7 count should keep increasing for control packets like icmp.

Additional information you deem important:

**Output of `show version`:**
 ```

SONiC Software Version: SONiC.20201231.91

```

**Attach debug file `sudo generate_dump`:**

```
(paste your output here)
```