Add test for bug in padding validation and get ready for challenge 18.

src/set3.rs

@@ -93,3 +93,7 @@ pub fn challenge17() {
     assert_eq!(roundtrip, cleartext);
     println!("[okay] Challenge 17: {}", roundtrip.to_utf8());
 }
+
+pub fn challenge18() {
+    println!("[xxxx] Challenge 18: ");
+}