Build four fixture libraries that export objc_getClass, sel_registerName,
objc_msgSend and the autorelease pool functions (complete, without
NSProcessInfo, returning no activity, without pool functions) and drive
the backend through them on any POSIX host. Check the exact message
sequence (begin with the user-initiated and latency-critical options,
retain, end, release), balanced pools on every path, a held activity ended
by the destructor, and unavailable and refused results.
