tests: update random_chars generator to map u8 to char

Fix 'value of type `char` cannot be built from `std::iter::Iterator<Item=u8>`' for split test.

refs: https://docs.rs/rand/0.8.4/rand/distributions/struct.Alphanumeric.html#example

tests/by-util/test_split.rs

@@ -22,6 +22,7 @@ use std::{
 fn random_chars(n: usize) -> String {
     thread_rng()
         .sample_iter(&rand::distributions::Alphanumeric)
+        .map(char::from)
         .take(n)
         .collect::<String>()
 }