uu+tests: use strip_prefix and strip_suffix

[thomasqueirozb]

This closes #2719

I opened this as a draft because of this:

coreutils/src/uucore/src/lib/features/mode.rs

Lines 167 to 173 in d72a135

	if let Some(second) = arg.chars().nth(1) {
	match second {
	'r' | 'w' | 'x' | 'X' | 's' | 't' | 'u' | 'g' | 'o' | '0'..='7' => {
	// TODO: use strip_prefix() once minimum rust version reaches 1.45.0
	*arg = arg[1..arg.len()].to_string();
	return true;
	}

I don't really see the benefit of using strip_prefix here since the prefix is already known to be there. IMO it would just add more complexity.

Once someone responds to this I will either remove the comment or implement it using strip_prefix

[jhscheer]

If I remember correctly, the first if was part of the work around because strip_prefix() wasn't available.
I think I had something like this in mind:

if let Some(arg_stripped) = arg.strip_prefix('-') {
    if let Some(second) = arg.chars().nth(1) {
        match second {
            'r' | 'w' | 'x' | 'X' | 's' | 't' | 'u' | 'g' | 'o' | '0'..='7' => {
                *arg = arg_stripped.to_string();
                return true; 
            }
            _ => {}
        }
    }
}

[thomasqueirozb]

@jhscheer I failed to look one line above of what I mentioned...

[miDeb]

Thanks!