[TSan] |ktf_worker_t::i| is unsynchronised

[iiSeymour]

TSan complains that this read of |ktf_worker_t::i| is unsynchronised wrt the stores on the other worker threads. Unlike the _atomic* family of functions the _sync* functions don't have a relaxed read without modification, so to emulate this behaviour I've used the existing __sync_fetch_and_add() with an increment of 0.