Add a test-only feature

Author: joegallo

server/src/main/java/org/elasticsearch/ingest/IngestFeatures.java

@@ -17,6 +17,7 @@
 public class IngestFeatures implements FeatureSpecification {
     private static final NodeFeature SIMULATE_INGEST_400_ON_FAILURE = new NodeFeature("simulate.ingest.400_on_failure", true);
     private static final NodeFeature INGEST_APPEND_COPY_FROM = new NodeFeature("ingest.append.copy_from", true);
+    private static final NodeFeature INGEST_APPEND_IGNORE_EMPTY_VALUES = new NodeFeature("ingest.append.ignore_empty_values", true);
 
     @Override
     public Set<NodeFeature> getFeatures() {
@@ -25,6 +26,6 @@ public Set<NodeFeature> getFeatures() {
 
     @Override
     public Set<NodeFeature> getTestFeatures() {
-        return Set.of(SIMULATE_INGEST_400_ON_FAILURE, INGEST_APPEND_COPY_FROM);
+        return Set.of(SIMULATE_INGEST_400_ON_FAILURE, INGEST_APPEND_COPY_FROM, INGEST_APPEND_IGNORE_EMPTY_VALUES);
     }
 }