AVRO-4062 [java] Add test for leading underscore in names

lang/java/idl/src/test/idl/input/leading_underscore.avdl

@@ -0,0 +1,26 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+Testing leading underscore for identifiers
+*/
+protocol _LeadingUnderscore {
+  record _TestRecord {
+    string _testField;
+  }
+}

lang/java/idl/src/test/idl/output/leading_underscore.avpr

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+{
+  "protocol" : "_LeadingUnderscore",
+  "doc":"Testing leading underscore for identifiers",
+  "types" : [
+    {
+      "type" : "record",
+      "name" : "_TestRecord",
+      "fields" : [
+        {
+          "name" : "_testField",
+          "type" : "string"
+        }
+      ]
+    }
+  ],
+  "messages" : {}
+}