Fix vkloadtests build issues

[MarkCallow]

Fixed issues include:

• failure to build with SDK having ASTC 3D extension due to syntax error.
• failure to copy the .spv files to the bundle's resources due to them not appearing in vkloadtests' SOURCES property.
• failure to build when VulkanSDK was not globally installed due to calling glslc instead of ${Vulkan_GLSLC_EXECUTABLE}.
• invalid enabling of validation layers at device creation. It must happen, and is/was, at instance creation.

Fixes #1235.