Update iproduct tests

Test name `iproduct1` starts with `i` because `product1` is for another kind of test.

Author: Philippe-Cholet

tests/macros_hygiene.rs

@@ -1,5 +1,6 @@
 #[test]
 fn iproduct_hygiene() {
+    let _ = itertools::iproduct!();
     let _ = itertools::iproduct!(0..6);
     let _ = itertools::iproduct!(0..6, 0..9);
     let _ = itertools::iproduct!(0..6, 0..9, 0..12);

tests/test_core.rs

@@ -18,6 +18,23 @@ use crate::it::Itertools;
 use core::iter;
 use itertools as it;
 
+#[test]
+fn product0() {
+    let mut prod = iproduct!();
+    assert_eq!(prod.next(), Some(()));
+    assert!(prod.next().is_none());
+}
+
+#[test]
+fn iproduct1() {
+    let s = "αβ";
+
+    let mut prod = iproduct!(s.chars());
+    assert_eq!(prod.next(), Some(('α',)));
+    assert_eq!(prod.next(), Some(('β',)));
+    assert!(prod.next().is_none());
+}
+
 #[test]
 fn product2() {
     let s = "αβ";