Clean up and unify exercises

exercises/06_move_semantics/move_semantics4.rs

@@ -1,20 +1,6 @@
-// move_semantics4.rs
-//
 // Refactor this code so that instead of passing `vec0` into the `fill_vec`
 // function, the Vector gets created in the function itself and passed back to
-// the main function.
-//
-// Execute `rustlings hint move_semantics4` 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]);
-}
+// the test function.
 
 // `fill_vec()` no longer takes `vec: Vec<i32>` as argument - don't change this!
 fn fill_vec() -> Vec<i32> {
@@ -25,3 +11,21 @@ fn fill_vec() -> Vec<i32> {
 
     vec
 }
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn move_semantics4() {
+        let vec0 = vec![22, 44, 66];
+
+        let vec1 = fill_vec(vec0);
+
+        assert_eq!(vec1, vec![22, 44, 66, 88]);
+    }
+}