[test gap] 'eth0' oper down causing BGP default route missing

[wenyiz2021]

Description shutting down of mgmt port (eth0) is causing default route deleted from APP_DB, this issue is seen 202012 and newer branches.

There is a test needed to ensure correct behavior on all branches.

Steps to reproduce the issue:

1. Reboot the system with 202012/202205 image
2. Learn BGP default route (0.0.0.0/0) from peers
3. Verify the default route is present in app_db with redis-cli hgetall "ROUTE_TABLE:0.0.0.0/0"
4. Ensure default route is also present in default table for eth0 ('ip route show table default')
5. Bring down eth0 from management switch (simulate oper-down on DUT, log in to mgmt switch through tlaloc, do set interfaces {port on mgmt switch} disable, then do commit), ensure eth0 is down by grep "eth0: Link is down" /var/log/syslog
6. Check for default route in APP_DB with redis-cli -n 0 hgetall "ROUTE_TABLE:0.0.0.0/0", or grep in swss/swss.rec for ROUTE_TABLE:0.0.0.0/0|DEL

Describe the results you received:

APP_DB default route is removed

Describe the results you expected: APP_DB default route must not be changed during eth0 link-up/down events

Additional information you deem important:

**Output of `show version`:**

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**Attach debug file `sudo generate_dump`:**

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