[Dualtor] IPv6 test coverage for dual tor cases

[prsunny]

Description

Current dualtor test cases for orchagent is covering only the IPv4 packet flow. This was the planned phase 1 delivery which is completed. As the next step, we need IPv6 traffic coverage

Steps to reproduce the issue: Enable IPv6 for the following cases:

1. https://github.com/Azure/sonic-mgmt/blob/master/tests/dualtor/test_orchagent_active_tor_downstream.py
2. https://github.com/Azure/sonic-mgmt/blob/master/tests/dualtor/test_orchagent_standby_tor_downstream.py
3. https://github.com/Azure/sonic-mgmt/blob/master/tests/dualtor/test_orchagent_slb.py

For this, the test must create IPv6 neighbor on both standby and active and send regular IPv6 traffic. For tunneled traffic, the expected packet would be IPv6 over IPv4.

Describe the results you received:

Describe the results you expected:

Additional information you deem important:

**Output of `show version`:**

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

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

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

[StormLiangMS]

confirmed with Longxiang, this one is done.