…vnet test cases (sonic-net#2873)

What I did

For outbound routing, I fix a bug that vnet name will lose in vnet_direct case.
The fvs from asic db is the dict type in test cases of dash vnet, for a dict will just for its keys.
Why I did it

The vnet name in vnet and vnet_direct cases are defined in a different field.
https://github.com/sonic-net/sonic-dash-api/blob/3f728d1bbf65d2e8c41bdc023d5c07702a7f848b/proto/route.proto#L30-L32
message VnetDirect {
    // destination vnet name if action_type is {vnet, vnet_direct}, a vnet other than eni's vnet means vnet peering
    string vnet = 1;
...
}

message Route {
    route_type.RoutingType action_type = 1;
    oneof Action {
        // destination vnet name if action_type is vnet,, a vnet other than eni's vnet means vnet peering
        string vnet = 2;
        // destination vnet name if action_type is vnet_direct,, a vnet other than eni's vnet means vnet peering
        route_lpm.VnetDirect vnet_direct = 3;
...
    }
  }
Add the items after fvs to fetch each items of fvs
How I verified it
Add a new test case and check tests.

$ sudo pytest --dvsname=vs --num-ports=32 test_dash_vnet.py  --pdb
====================================== test session starts ======================================
platform linux -- Python 3.8.10, pytest-7.2.2, pluggy-1.0.0
rootdir: /home/zegan/workspace/sonic-swss/tests
plugins: flaky-3.7.0
collected 8 items

test_dash_vnet.py ........                                                                [100%]

======================================= warnings summary ========================================


Signed-off-by: Ze Gan <[email protected]>