New J2K: add tests for some obsolete J2K issues

nj2k/testData/newJ2k/issues/kt-19604.java

@@ -0,0 +1,11 @@
+// RUNTIME_WITH_FULL_JDK
+
+import java.util.HashMap;
+
+public class TestMethodReference {
+    private HashMap<String, String> hashMap = new HashMap<>();
+
+    public void update(String key, String msg) {
+        hashMap.merge(key, msg, String::concat);
+    }
+}