avoid transient changes to MastershipStore from being posted as events

Change-Id: Id033cf50f865e44439138f5b3814ebaedb832b73
diff --git a/core/net/src/test/java/org/onlab/onos/net/device/impl/DeviceManagerTest.java b/core/net/src/test/java/org/onlab/onos/net/device/impl/DeviceManagerTest.java
index d079378..25fa922 100644
--- a/core/net/src/test/java/org/onlab/onos/net/device/impl/DeviceManagerTest.java
+++ b/core/net/src/test/java/org/onlab/onos/net/device/impl/DeviceManagerTest.java
@@ -278,6 +278,11 @@
             deviceReceived = device;
             roleReceived = newRole;
         }
+
+        @Override
+        public boolean isReachable(Device device) {
+            return false;
+        }
     }
 
     private static class TestListener implements DeviceListener {