More reliable testing of GCD Queues

See https://devforums.apple.com/message/710745\#714753 for a lengthy discussion on why this should be done.

I was always scared about what I'd done, since I'd read that `dispatch_get_current_queue()` cannot be relied upon (it\s even been deprecated on iOS since Apple don't want people using it for the wrong things - for debugging only) so I'm glad I've found an alternative :)