Add toString for NotifyingNodeEntry -- we lost this in commit 8be7fd0bb7cc8f0819f23ef4bbf6328472110db7 when NotifyingNodeEntry stopped inheriting from InMemoryNodeEntry.

--
MOS_MIGRATED_REVID=124960351
diff --git a/src/test/java/com/google/devtools/build/skyframe/NotifyingGraph.java b/src/test/java/com/google/devtools/build/skyframe/NotifyingGraph.java
index bf3ab1b..22dbc35 100644
--- a/src/test/java/com/google/devtools/build/skyframe/NotifyingGraph.java
+++ b/src/test/java/com/google/devtools/build/skyframe/NotifyingGraph.java
@@ -15,6 +15,7 @@
 
 import com.google.common.base.Function;
 import com.google.common.base.Joiner;
+import com.google.common.base.MoreObjects;
 import com.google.common.collect.Maps;
 import com.google.common.collect.Maps.EntryTransformer;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;
@@ -279,5 +280,10 @@
           myKey, EventType.GET_ALL_DIRECT_DEPS_FOR_INCOMPLETE_NODE, Order.BEFORE, this);
       return super.getAllDirectDepsForIncompleteNode();
     }
+
+    @Override
+    public String toString() {
+      return MoreObjects.toStringHelper(this).add("delegate", getThinDelegate()).toString();
+    }
   }
 }