Add missing test for demangle for OSX

[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:

rustc-demangle/src/v0.rs

Lines 48 to 51 in af38dc6

	} else if s.len() > 3 && s.starts_with("__R") {
	// On OSX, symbols are prefixed with an extra _
	inner = &s[3..];
	} else {

Thanks again for reviewing.