j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/issues/kt-820-string.java

@@ -0,0 +1,7 @@
+//file
+class Test {
+    public static String toFileSystemSafeName(String name) {
+        int size = name.length();
+        return name;
+    }
+}