Issue Description:

I was trying to migrate some integration testing code so it could be Linux compatible. I was surprised to discover that the open source corelibs-xctest does not implement XCTAttachment nor XCTActivity. I'm sure there's some sort of technical or procedural issue that I'm not aware of that would explain this omission, but it surprised me nonetheless.

I've worked around it in a rudimentary way for my purposes, so this isn't blocking, but I'd be interested in seeing some of these less commonly-used XCTest features come to Linux.