Tests for VNET route precedence over BGP learnt route.

[siqbal1986]

Description of PR

These tests check the Vnet route precedence over bgp learnt route.

Summary:
Fixes # (issue)

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• Test case(new/improvement)

Back port request

• 202012
• 202205
• 202305
• 202311
• 202405

Approach

What is the motivation for this PR?

Currently if a route is learnt via BGP and added into the hardware, adding a VNET route results in failure. In addition due to a bug in VnetOrch, we start advertising the failed route, This prompts the BGP to remove the learnt route in favor of the local route. Since the Vnet Orch doesn't retry adding the Vnet route, This results in no route being present in the Hardware.

How I verified it

The fix is in PR sonic-net/sonic-swss#3345
These tests cover various scenario in which VNET and BGP routes are added and removed in different order

How did you do it?

How did you verify/test it?

Any platform specific information?

Cisco-8000 and mlnx.

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

Documentation

There are 5 differnet scenarios which are checked. Each scenario is tested with with the following options.
Encap types [v4_inv4, v6_inV4]
Monitor Type [BFD, Custom]
Init NH state( BFD/monitor sessions for nexthops are initially up or not.)

1. test_vnet_route_after_bgp
   ADD BGP ROUTE on TOR
   Add VNET route
   Configure monitor (BFD or custom) with nexthop state (UP)
   Test with traffic
   Remove VNET route
   Remove BGP route
2. test_vnet_route_before_bgp_after_ep_up
   Add VNET route
   Configure monitor (BFD or custom) with nexthop state (UP)
   Add BGP ROUTE on TOR
   Test with traffic
   Remove VNET ROUTE
   Remove BGP route
3. test_vnet_route_bgp_removal_before_ep
   ADD BGP ROUTE on TOR
   Add VNET route
   Remove BGP route
   Configure monitor (BFD or custom) with nexthop state (UP)
   Test with traffic
   Remove VNET route
4. test_vnet_route_after_bgp_with_early_bgp_removal
   Add VNET route
   Add BGP ROUTE on TOR
   Configure monitor (BFD or custom) with nexthop state (UP)
   Test with traffic
   Remove BGP route
   Test with traffic
   Remove VNET route
5. test_vnet_route_after_bgp_multi_flap
   ADD BGP ROUTE on TOR
   Add VNET route
   Configure monitor (BFD or custom) with nexthop state (UP)
   Test with traffic
   flap the bfd/monitor sessions.
   Test with traffic
   Remove VNET route
   Remove BGP route

Note: Some scenarios are skipped because they are not valid.

[mssonicbld]

The pre-commit check detected issues in the files touched by this pull request.
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Detailed pre-commit check results:
trim trailing whitespace.................................................Failed
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Fixing tests/vxlan/test_vnet_bgp_route_precedence.py

fix end of files.........................................................Failed
- hook id: end-of-file-fixer
- exit code: 1
- files were modified by this hook

Fixing tests/vxlan/test_vnet_bgp_route_precedence.py

check yaml...........................................(no files to check)Skipped
check for added large files..............................................Passed
check python ast.........................................................Passed
flake8...................................................................Failed
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