Test plan for next hop split

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)

Author: TACappleman

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 IPv4 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