Converter tests:

Change test data files extension to "java"

j2k/tests/testData/ast/function/overrideAndOpen.java

@@ -0,0 +1,12 @@
+//file
+class A {
+  void foo() {}
+}
+
+class B extends A {
+  void foo() {}
+}
+
+class C extends B {
+  void foo() {}
+}