Error message was:
[mvk-info] MoltenVK version 0.18.2. Vulkan version 1.0.51.
[***MoltenVK ERROR***] VK_ERROR_INITIALIZATION_FAILED: On-screen rendering requires a view that is backed by a layer of type CAMetalLayer.
2017-08-28 02:17:29.579 testvulkan[95627:1716939] ERROR: SDL_Vulkan_CreateSurface(): vkCreateMacOSSurfaceMVK failed: VK_ERROR_INITIALIZATION_FAILED