[radv_ipv6_ra]: Fix the hardcode interface for IPv6 advertisement interface

[Pterosaur]

Description of PR

Summary:
Fixes # (issue)

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• New Test case
  • Skipped for non-supported platforms
• Test case improvement

Back port request

• 202012
• 202205
• 202305
• 202311
• 202405
• 202411

Approach

What is the motivation for this PR?

In some testbeds, eth1 isn't always present. This PR is to enhance the test case to support the testbed without eth1.

How did you do it?

Replace the ptf_port_index as IPv6 advertisement interface which will be always present in the testbed.

How did you verify/test it?

radv/test_radv_ipv6_ra.py::test_radv_router_advertisement PASSED                                      [ 25%]
radv/test_radv_ipv6_ra.py::test_solicited_router_advertisement PASSED                                [ 50%]
radv/test_radv_ipv6_ra.py::test_unsolicited_router_advertisement_with_m_flag PASSED       [ 75%]
radv/test_radv_ipv6_ra.py::test_solicited_router_advertisement_with_m_flag PASSED           [ 100%]

Any platform specific information?

Supported testbed topology if it's a new test case?

Documentation

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[r12f]

Lgtm! Thanks Ze!