simple-linked-list: Use range instead of vec!

Clippy suggested:
useless use of `vec!`

https://rust-lang-nursery.github.io/rust-clippy/master/index.html#useless_vec

Author: petertseng

exercises/simple-linked-list/tests/simple-linked-list.rs

@@ -82,7 +82,7 @@ fn test_reverse() {
 fn test_into_vector() {
     let mut v = Vec::new();
     let mut s = SimpleLinkedList::new();
-    for i in vec![1, 2, 3] {
+    for i in 1..4 {
         v.push(i);
         s.push(i);
     }