fix multifield tests failure

Author: Samiul-TheSoccerFan

server/src/main/java/org/elasticsearch/index/mapper/FieldMapper.java

@@ -451,7 +451,12 @@ public int getTotalFieldsCount() {
 
     @Override
     public List<FieldMapper> getSourceFields() {
-        return List.of(this);
+        List<FieldMapper> fields = new ArrayList<>();
+        for (FieldMapper mapper : builderParams.multiFields.mappers) {
+            fields.addAll(mapper.getSourceFields());
+        }
+        fields.add(this);
+        return fields;
     }
 
     public Map<String, NamedAnalyzer> indexAnalyzers() {