tests: Update layer doc to reflect recent test changes
tests: Update layer doc to reflect recent test changes

Change-Id: I52ed15e0455b88ad7cf7474baa1d1790a13a86f0

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools