Adding topology test suite.
diff --git a/core/trivial/src/main/java/org/onlab/onos/net/trivial/link/impl/SimpleLinkManager.java b/core/trivial/src/main/java/org/onlab/onos/net/trivial/link/impl/SimpleLinkManager.java
index a817bb6..4ea1753 100644
--- a/core/trivial/src/main/java/org/onlab/onos/net/trivial/link/impl/SimpleLinkManager.java
+++ b/core/trivial/src/main/java/org/onlab/onos/net/trivial/link/impl/SimpleLinkManager.java
@@ -45,7 +45,7 @@
 
     private final Logger log = getLogger(getClass());
 
-    private final AbstractListenerRegistry<LinkEvent, LinkListener>
+    protected final AbstractListenerRegistry<LinkEvent, LinkListener>
             listenerRegistry = new AbstractListenerRegistry<>();
 
     private final SimpleLinkStore store = new SimpleLinkStore();