Adding support for writing metadata instruction.
Needed by Centec driver.

Change-Id: I705015c4d7ac881c273a01b36b05d39d9cd9ba87
diff --git a/core/net/src/test/java/org/onosproject/net/flow/impl/FlowRuleManagerTest.java b/core/net/src/test/java/org/onosproject/net/flow/impl/FlowRuleManagerTest.java
index 1113b2b..5097bb8 100644
--- a/core/net/src/test/java/org/onosproject/net/flow/impl/FlowRuleManagerTest.java
+++ b/core/net/src/test/java/org/onosproject/net/flow/impl/FlowRuleManagerTest.java
@@ -21,6 +21,7 @@
 import com.google.common.collect.Sets;
 import com.google.common.util.concurrent.ListenableFuture;
 import com.google.common.util.concurrent.MoreExecutors;
+
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
@@ -58,6 +59,7 @@
 import org.onosproject.net.flow.criteria.Criterion;
 import org.onosproject.net.flow.instructions.Instruction;
 import org.onosproject.net.flow.instructions.Instructions;
+import org.onosproject.net.flow.instructions.Instructions.MetadataInstruction;
 import org.onosproject.net.provider.AbstractProvider;
 import org.onosproject.net.provider.ProviderId;
 import org.onosproject.store.trivial.impl.SimpleFlowRuleStore;
@@ -595,6 +597,11 @@
             return false;
         }
 
+        @Override
+        public MetadataInstruction writeMetadata() {
+            return null;
+        }
+
     }
 
     public class TestApplicationId extends DefaultApplicationId {