build: Fix parallel build of layers tests
build: Fix parallel build of layers tests

Make json copy targets depend on layer targets to first create destination directory.
Make header generation go through a custom target.

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers