Add missing unit test for i128 positive

[CXWorks]

Hi,

Thanks for your time & patience to review this PR.

We are researchers focusing on Rust unit tests by LLM. By examine the existing code, we found a unit test can be added to improve the repo's overall unit test coverage(this project is already been well tested).
The code region to cover is:

itoa/src/lib.rs

Lines 239 to 241 in 0d040f4

	let n = if is_nonnegative {
	self as u128
	} else {

Thanks again for reviewing.

[dtolnay]

Thanks!