test: add first_max_element check for 1-elem lists

This is not needed currently as cost is always 8 elements long but will guard against
unexpected breakage should the function be used on single element lists in the future.

src/cdef.rs

@@ -308,6 +308,7 @@ pub(crate) mod rust {
       assert_eq!(first_max_element(&[-1, -1, 1, 2, 3, 4, 6, 6]), (6, 6));
       assert_eq!(first_max_element(&[-1, -1, 1, 2, 3, 4, 7, 6]), (6, 7));
       assert_eq!(first_max_element(&[0, 0]), (0, 0));
+      assert_eq!(first_max_element(&[0]), (0, 0));
     }
   }
 }