Clean up and unify exercises

exercises/06_move_semantics/move_semantics3.rs

@@ -1,22 +1,26 @@
-// move_semantics3.rs
-//
 // Make me compile without adding new lines -- just changing existing lines! (no
 // lines with multiple semicolons necessary!)
-//
-// Execute `rustlings hint move_semantics3` or use the `hint` watch subcommand
-// for a hint.
-
-#[test]
-fn main() {
-    let vec0 = vec![22, 44, 66];
-
-    let vec1 = fill_vec(vec0);
-
-    assert_eq!(vec1, vec![22, 44, 66, 88]);
-}
 
 fn fill_vec(vec: Vec<i32>) -> Vec<i32> {
     vec.push(88);
 
     vec
 }
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn move_semantics3() {
+        let vec0 = vec![22, 44, 66];
+
+        let vec1 = fill_vec(vec0);
+
+        assert_eq!(vec1, vec![22, 44, 66, 88]);
+    }
+}