Bug fix in testing of return code of sem_timedwait
Bug fix in testing of return code of sem_timedwait

Code was not properly handling a return code of 0 from sem_timedwait
(it would interpret it as a timeout).  Change logic of test to match
that used after the other call to sem_timedwait in this file.

frankeh/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch