Async testing technique with semaphores based on http://stackoverflow.com/a/28285276/1672161

semaphore-async-test.swift

let semaphore = DispatchSemaphore(value: 0)

asyncFunction() { result in
    XCTAssert(result)
    semaphore.signal 
}

while semaphore.wait(timeout: .now()) == .timedOut {
    RunLoop.current.run(until: Date(timeIntervalSinceNow: 3))
}