Added test case to verify AZNG Migration for T2 topology

[abdosi]

What I did:

Added new test case to verify AZNG Migration for T2 topology. Test will run for MSFT internal image only.
Test verify after each step of AZNG Migration route-map functionality is as expected.
Test cover both v4 and v6 scenarios.
MSFT ADO: 29397531

How I verify:
Test passed on T2 topology.

[mssonicbld]

The pre-commit check detected issues in the files touched by this pull request.
The pre-commit check is a mandatory check, please fix detected issues.

Detailed pre-commit check results:
trim trailing whitespace.................................................Failed
- hook id: trailing-whitespace
- exit code: 1
- files were modified by this hook

Fixing tests/bgp/test_bgp_azng_migration.py

fix end of files.........................................................Passed
check yaml...........................................(no files to check)Skipped
check for added large files..............................................Passed
check python ast.........................................................Passed
flake8...................................................................Failed
- hook id: flake8
- exit code: 1

tests/bgp/test_bgp_azng_migration.py:13:1: E302 expected 2 blank lines, found 1
tests/bgp/test_bgp_azng_migration.py:20:17: E221 multiple spaces before operator
tests/bgp/test_bgp_azng_migration.py:46:8: E111 indentation is not a multiple of 4
tests/bgp/test_bgp_azng_migration.py:88:56: E222 multiple spaces after operator
tests/bgp/test_bgp_azng_migration.py:91:56: E222 multiple spaces after operator
...
[truncated extra lines, please run pre-commit locally to view full check results]

To run the pre-commit checks locally, you can follow below steps:

1. Ensure that default python is python3. In sonic-mgmt docker container, default python is python2. You can run
   the check by activating the python3 virtual environment in sonic-mgmt docker container or outside of sonic-mgmt
   docker container.
2. Ensure that the pre-commit package is installed:

sudo pip install pre-commit

3. Go to repository root folder
4. Install the pre-commit hooks:

pre-commit install

5. Use pre-commit to check staged file:

pre-commit

6. Alternatively, you can check committed files using:

pre-commit run --from-ref <commit_id> --to-ref <commit_id>

[mssonicbld]

Cherry-pick PR to 202405: #15779