Fix decap test on FT2 topo

[bingwang-ms]

Description of PR

Summary:
This PR is to fix decap test on FT2 topo.

The test gets stuck on FT2 because all the ports are in nexthop for default route. Hence the check in function get_src_and_exp_ports runs into dead loop.

This PR addressed that issue by checking if the route points to default route, and ignore the check src_port in exp_port_list if that's the case.

This PR depends on #18442

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?

This PR is to fix decap test on FT2 topo.

How did you do it?

This PR addressed that issue by checking if the route points to default route, and ignore the check src_port in exp_port_list if that's the case.

How did you verify/test it?

The change is verified on a FT2 topo.

collected 4 items                                                                                                                                                                                                             

decap/test_decap.py::test_decap[ttl=pipe, dscp=uniform, vxlan=disable] 
-------------------------------------------------------------------------------------------------------- live log call --------------------------------------------------------------------------------------------------------
23:29:27 ptf_runner.get_dut_type                  L0053 WARNING| DUT type file doesn't exist.
PASSED                                                                                                                                                                                                                  [ 25%]
decap/test_decap.py::test_decap[ttl=pipe, dscp=uniform, vxlan=set_unset] SKIPPED (Not supported on backend, T2 topologies , broadcom platforms before 202012 release, marvell-teralynx, x86_64-8111_32eh_o-r0 platform. Skip on mellanox 202412 release. Skip on 7260CX3 T1 topo in 202305 release) [ 50%]
decap/test_decap.py::test_decap[ttl=pipe, dscp=pipe, vxlan=disable] SKIPPED (Not supported on broadcom after 201911 release and cisco-8000 all releases and marvell asics)                                              [ 75%]
decap/test_decap.py::test_decap[ttl=pipe, dscp=pipe, vxlan=set_unset] SKIPPED (Not supported on broadcom after 201911 release, and cisco-8000 all releases and marvell asics)                                           [100%]

Any platform specific information?

No.

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

Not a new test.

Documentation

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mssonicbld]

Cherry-pick PR to msft-202503: Azure/sonic-mgmt.msft#476