Add insufficient length test

It seems to me like if we're already testing for an overly long slice, we might as well check for one that is too short as well.

exercises/conversions/try_from_into.rs

@@ -127,4 +127,10 @@ mod tests {
         let v = vec![0, 0, 0, 0];
         let _ = Color::try_from(&v[..]).unwrap();
     }
+    #[test]
+    #[should_panic]
+    fn test_slice_insufficient_length() {
+        let v = vec![0, 0];
+        let _ = Color::try_from(&v[..]).unwrap();
+    }
 }