feat: support eos neighbor for nlri test (sonic-net#17659)

Description of PR
Support EOS neighbor for bgp/test_ipv6_nlri_over_ipv4.py test.

Summary:
Fixes # (issue) Microsoft ADO 28837595

Approach
What is the motivation for this PR?
We wanted to support EOS neighbor for bgp/test_ipv6_nlri_over_ipv4.py test to increase the coverage for T2 test.

How did you do it?
How did you verify/test it?
I ran the updated code and can confirm it's working well: https://elastictest.org/scheduler/testplan/67e0e19fbfeec1f856658690

co-authorized by: jianquanye@microsoft.com

Author: cyw233