Improve core::num coverage

Author: pvdrz

library/coretests/tests/num/mod.rs

@@ -111,6 +111,13 @@ fn from_str_issue7588() {
     assert_eq!(s, None);
 }
 
+#[test]
+#[should_panic = "radix must lie in the range `[2, 36]`"]
+fn from_ascii_radix_panic() {
+    let radix = 1;
+    let _parsed = u64::from_str_radix("12345ABCD", radix);
+}
+
 #[test]
 fn test_int_from_str_overflow() {
     test_parse::<i8>("127", Ok(127));