tests(date): Add tests for #6392

These tests check that days and month with a single digit are correctly handled.

tests/by-util/test_date.rs

@@ -420,6 +420,21 @@ fn test_invalid_date_string() {
         .stderr_contains("invalid date");
 }
 
+#[test]
+fn test_date_one_digit_date() {
+    new_ucmd!()
+        .arg("-d")
+        .arg("2000-1-1")
+        .succeeds()
+        .stdout_contains("Sat Jan  1 00:00:00 2000");
+
+    new_ucmd!()
+        .arg("-d")
+        .arg("2000-1-4")
+        .succeeds()
+        .stdout_contains("Tue Jan  4 00:00:00 2000");
+}
+
 #[test]
 fn test_date_overflow() {
     new_ucmd!()