j2k: flatten test cases and testData directory structure

Move j2k/test/tests -> j2k/tests, j2k/test/testData -> j2k/testData

j2k/testData/fileOrElement/enum/enumConstantIsNotNull.java

@@ -0,0 +1,8 @@
+//class
+enum E {
+  FOO;
+
+  void foo() {
+    FOO.toString();
+  }
+}