[ONOS-3222] Implement toString method for each Packet class

Change-Id: I17d72338d4202117d08b3dca9463be35a87a0c1e
diff --git a/utils/misc/src/test/java/org/onlab/packet/IGMPTest.java b/utils/misc/src/test/java/org/onlab/packet/IGMPTest.java
index a25f721..7634bbf 100644
--- a/utils/misc/src/test/java/org/onlab/packet/IGMPTest.java
+++ b/utils/misc/src/test/java/org/onlab/packet/IGMPTest.java
@@ -93,4 +93,12 @@
         IGMP igmp = deserializer.deserialize(data, 0, data.length);
         assertTrue(igmp.equals(igmpMembership));
     }
+
+    /**
+     * Tests toString.
+     */
+    @Test
+    public void testToStringIgmp() throws Exception {
+        // TODO: add toString unit test
+    }
 }