Add rif loopback action test plan

docs/testplan/ip-interface/rif_loopback_action_testplan.md

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+# Ip Interface Loopback Action Test Plan
+
+## Related documents
+
+| **Document Name** | **Link** |
+|-------------------|----------|
+| rif_loopback_action_hld | [https://github.com/sonic-net/SONiC/doc/ip-interface/loopback-action/ip-interface-loopback-action-design.md]|
+
+
+## Overview
+RIF loopback action is a feature which allows user to change the way router handles routed packets with egress port equal to ingress port.
+
+1. When RIF loopback action is configured to drop, routed packet ingress port and egress port can not be equal. In other words, if routed packet entered the router from a port and destined to egress from the same one, it will be dropped.
+2. When RIF loopback action is configured to forward, routed packet ingress port and egress port can be equal.
+3. Default loopback action in SAI is forward. SONiC will rely on SAI default (the motivation here is not to change the behaviour if other SAI vendors have different default).
+
+
+## Requirements
+
+#### The Rif loopback action is support on following ip interface:
+1. Ethernet interface
+2. Vlan interface
+3. PortChannel interface
+4. Ethernet sub-interface
+5. PortChannel sub-interface
+
+#### This feature will support the following commands:
+
+1. config: set RIF loopback action
+2. show: display loopback action of all RIFs
+
+#### This feature will provide error handling for the next situations:
+
+1. Invalid action
+2. Invalid interface
+3. Non ip interface
+
+### Scope
+
+The test is to verify the loopback action can be configured, and the loopback traffic can be forwarded/droppped as expect according to the action configured on the ip interface.   
+
+### Scale / Performance
+
+No scale test related
+
+### Related **DUT** CLI commands
+
+#### Config
+The following command can be used to configure loopback action on L3 interface:
+```
+config interface ip loopback-action <interface-name> <action>
+```
+action options: "drop", "forward"
+
+Examples:
+```
+config interface ip loopback-action Ethernet248 drop
+config interface ip loopback-action Vlan100 forward
+config interface ip loopback-action PortChannel1 drop
+config interface ip loopback-action Ethernet0.10 drop
+config interface ip loopback-action Po10.10 drop
+```
+
+#### Show
+The following command can be used to show loopback action:
+```
+show ip interfaces loopback-action 
+```
+Example:
+```
+show ip interfaces loopback-action
+Interface     Action      
+------------  ----------  
+Ethernet248   drop     
+Vlan100       forward     
+PortChannel1  drop
+Ethernet0.10  drop
+Po10.10       drop
+```
+### Related DUT configuration files
+
+```
+"INTERFACE": {
+    "Ethernet248": {
+        "loopback_action": "drop"
+    },
+},
+"PORTCHANNEL_INTERFACE": {
+    "PortChannel1": {
+        "loopback_action": "drop"
+    },
+},
+"VLAN_INTERFACE": {
+    "Vlan100": {
+        "loopback_action": "drop"
+    },
+},
+"VLAN_SUB_INTERFACE": {
+    "Ethernet0.10": {
+        "admin_status": "up",
+        "loopback_action": "drop",
+        "vlan": "10"
+    },
+    "Po10.10": {
+        "loopback_action": "drop",
+        "admin_status": "up",
+        "vlan": "10"
+    }
+}
+```
+### Supported topology
+The test will be supported on any topology
+
+
+## Test cases
+
+### Test cases #1 - Verify the loopback action can be configured and worked as expect,the interface type will cover all the 5 types of ip interface.
+1. Verify the default action with sending traffic: 
+   - Verify the traffic will be forwarded as expected.
+   - The RIF TX_ERR counter will not increase as expected
+2. Configure the loopback action to drop on the interface with cli command
+3. Verify the the loopback action is configured correctly with show cli command.
+4. Send traffic to do the validation: 
+   - Verify the traffic will be dropped as expected.
+   - The RIF TX_ERR counter will increase as expected
+5. Change it back to forward
+6. Verify the the loopback action is configured correctly with show cli command.
+7. Send traffic to do the validation
+   - Verify the traffic will be forwarded as expected.
+   - The RIF TX_ERR counter will not increase.
+
+### Test cases #2 - Disable/Enable the interface and check if the loopback action still work, the interface type will cover all types of ip interface except vlan interface.
+1. Configure the loopback action on the interface, some to drop, some to forward.
+2. Disable the interfaces
+3. Enable the interfaces back
+4. Verify the loopback action is not changed with show cli command.
+5. Send traffic to do the validation: 
+   - Verify the traffic will be dropped/forwarded as expected.
+   - The RIF TX_ERR counter will increase on the interface that the loopback action is configured to drop
+   - The RIF TX_ERR counter will not increase on the interface that the loopback action is configured to forward
+
+### Test cases #3 - Verify loopback action is saved after reboot(config reload, reboot, fast-reboot, warm-reboot), the interface type will cover all the 5 types of ip interface.
+1. Configure the loopback action on the interface, some to drop, some to forward.
+2. Config save -y
+3. Do reboot(config reload/reboot/fast-reboot/warm-reboot)
+4. Verify the loopback action is saved correctly with show cli command.
+5. Send traffic to do the validation: 
+   - Verify the traffic will be dropped as expected.
+   - The RIF TX_ERR counter will increase on the interface that the loopback action is configured to drop
+   - The RIF TX_ERR counter will not increase on the interface that the loopback action is configured to forward
+