tests: add CFAttributedString accessor tests

[DTW-Thalion]

Adds accessor coverage for CFAttributedString: creating an attributed string and reading its type ID, string, length, attributes and single attribute with effective range, copying it, creating a mutable copy that keeps the string and attributes, and creating a mutable string then replacing whole and partial character ranges.

Verified against Apple CoreFoundation.

[HendrikHuebner]

Try rebasing to include the new CI workflows please.

[DTW-Thalion]

Rebased onto current master so this picks up the new CI workflows.

Doing the rebase surfaced a pre-existing bug in master that breaks CI for every branch, so I've opened #155 to fix it. In short: the new fast-enumeration tests (Tests/CFDictionary/fast_enumeration.m and Tests/CFSet/fast_enumeration.m, added in #117) import <Foundation/NSNumber.h>. GNUstep declares NSNumber in Foundation/NSValue.h and ships no standalone NSNumber.h, and the macOS compatibility compile (-framework Foundation) doesn't expose one as an includable header either, so both files fail to build on both runners with:

fast_enumeration.m:2:9: fatal error: 'Foundation/NSNumber.h' file not found

A failed build makes make check exit non-zero, which fails the whole Linux and Apple jobs on every PR. #155 switches those two imports to the <Foundation/Foundation.h> umbrella, which resolves NSNumber on both platforms. Once #155 lands, the rebased PRs should go green via the merge-ref build.

cc @stefanbidi @HendrikHuebner