Change if-else exercise to require an if-else (#238)

exercises/02_basic_calculator/03_if_else/src/lib.rs

@@ -1,24 +1,36 @@
-/// Return `true` if `n` is even, `false` otherwise.
-fn is_even(n: u32) -> bool {
+/// Return `12` if `n` is even,
+/// `13` if `n` is divisible by `3`,
+/// `17` otherwise.
+fn magic_number(n: u32) -> u32 {
     todo!()
 }
 
 #[cfg(test)]
 mod tests {
-    use crate::is_even;
+    use crate::magic_number;
 
     #[test]
     fn one() {
-        assert!(!is_even(1));
+        assert_eq!(magic_number(1), 17);
     }
 
     #[test]
     fn two() {
-        assert!(is_even(2));
+        assert_eq!(magic_number(2), 12);
+    }
+
+    #[test]
+    fn six() {
+        assert_eq!(magic_number(6), 12);
+    }
+
+    #[test]
+    fn nine() {
+        assert_eq!(magic_number(9), 13);
     }
 
     #[test]
     fn high() {
-        assert!(!is_even(231));
+        assert_eq!(magic_number(233), 17);
     }
 }