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:
@@ -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>()
}
