Test plan for next hop split (#3459)

What is the motivation for this PR?
This is the test plan for the next hop group split enhancement, as covered in PR 712 (sonic-net/SONiC#712)

docs/testplan/Next-hop-split-test-plan.md

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+# **Next Hop Split Test Plan**
+
+ - [Introduction](#introduction)
+ - [Scope](#scope)
+ - [Test Setup](#test-setup)
+ - [Test Cases](#test-cases)
+
+# Introduction
+
+This is the test plan for the next hop group split enhancement
+
+The PR covered in this test plan is [Next hop group split HLD PR 712](https://github.com/Azure/SONiC/pull/712)
+
+## Scope
+
+This test plan covers a new method of programming routes into APP_DB, where the next hop information is included in a separate NEXT_HOP_GROUP_TABLE referenced by the ROUTE_TABLE and LABEL_ROUTE_TABLE.
+
+There is no support in the BGP container/fpmsyncd for this feature, so routes and next hop groups are all programmed directly into APP_DB by the test scripts.
+
+# Test Setup
+
+These test cases will be run in the T0 topology.
+
+# Test Cases
+
+The methods used to support these test cases will be very similar to the methods in test_static_route.py (for adding IP addresses and routes and for checking traffic).
+
+## Test Case 1. IPv4 routes
+
+### Test Objective
+Verify that IPv4 routes and next hop groups can be programmed and cause the correct data plane behaviour.
+
+### Test Steps
+* Create an IPv4 next hop group with a single next hop
+* Create an IPv4 prefix route referencing the next hop
+* Create an IPv4 next hop group with multiple next hops
+* Update the route to reference the new next hop group
+
+### Pass/Fail Criteria
+*  After the route is created verify that traffic for the prefix is routed via the next hop
+*  After the route is updated verify that traffic for the prefix is routed via a next hop in the group
+
+## Test Case 2. IPv6 routes
+
+### Test Objective
+Verify that IPv6 routes and next hop groups can be programmed and cause the correct data plane behaviour.
+
+### Test Steps
+* Create an IPv6 next hop group with a single next hop
+* Create an IPv6 prefix route referencing the next hop
+* Create an IPv6 next hop group with multiple next hops
+* Update the route to reference the new next hop group
+
+### Pass/Fail Criteria
+*  After the route is created verify that traffic for the prefix is routed via the next hop
+*  After the route is updated verify that traffic for the prefix is routed via a next hop in the group
+
+## Test Case 3. MPLS routes
+
+### Test Objective
+Verify that MPLS routes and next hop groups can be programmed and cause the correct data plane behaviour.
+
+### Test Steps
+* Create an IPv4 next hop group with a single labeled next hop
+* Create a label route referencing the next hop
+* Create an IPv4 next hop group with multiple labeled next hops
+* Update the label route to reference the new next hop group
+
+### Pass/Fail Criteria
+*  After the route is created verify that traffic for the prefix is routed via the next hop
+*  After the route is updated verify that traffic for the prefix is routed via a next hop in the group