Add factorial of zero test in standand_library_types iterators4

exercises/standard_library_types/iterators4.rs

@@ -18,6 +18,11 @@ pub fn factorial(num: u64) -> u64 {
 mod tests {
     use super::*;
 
+    #[test]
+    fn factorial_of_0() {
+        assert_eq!(1, factorial(0));
+    }
+
     #[test]
     fn factorial_of_1() {
         assert_eq!(1, factorial(1));