ONOS-4075 TestDistributedSet implementation and Junit tests.

Change-Id: I77ea0beb5a1adbdbf8632c599b631d728455aa9a
diff --git a/core/api/src/test/java/org/onosproject/store/service/TestStorageService.java b/core/api/src/test/java/org/onosproject/store/service/TestStorageService.java
index 9b52a30..eb330fb 100644
--- a/core/api/src/test/java/org/onosproject/store/service/TestStorageService.java
+++ b/core/api/src/test/java/org/onosproject/store/service/TestStorageService.java
@@ -30,7 +30,7 @@
 
     @Override
     public <E> DistributedSetBuilder<E> setBuilder() {
-        throw new UnsupportedOperationException("setBuilder");
+        return TestDistributedSet.builder();
     }
 
     @Override