Add GossipLinkStoreTest

Change-Id: If3d9777583a38d911b19bb1bc50212ccb621918a
diff --git a/core/store/dist/src/main/java/org/onlab/onos/store/link/impl/GossipLinkStore.java b/core/store/dist/src/main/java/org/onlab/onos/store/link/impl/GossipLinkStore.java
index 29669af..351581d 100644
--- a/core/store/dist/src/main/java/org/onlab/onos/store/link/impl/GossipLinkStore.java
+++ b/core/store/dist/src/main/java/org/onlab/onos/store/link/impl/GossipLinkStore.java
@@ -122,7 +122,7 @@
     @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
     protected ClusterService clusterService;
 
-    private static final KryoSerializer SERIALIZER = new KryoSerializer() {
+    protected static final KryoSerializer SERIALIZER = new KryoSerializer() {
         @Override
         protected void setupKryoPool() {
             serializerPool = KryoNamespace.newBuilder()