j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/issues/kt-871.java

@@ -0,0 +1,8 @@
+//file
+package demo;
+
+class Program {
+  public static void main(String[] args) {
+    System.out.println("Halo!");
+  }
+}