gitlab-ci: x86_test-base image as common base for x86_test-gl/vk

Making use of the relatively recent FDO_BASE_IMAGE feature of the templates, the x86_test-base image contents are shared as a separate layer by the x86_test-gl/vk images (meaning the former only needs to be downloaded once for either or both of the latter). This should be more efficient in terms of overall network bandwidth and storage, in particular if the base image changes less often than the -gl/vk ones.