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.

v2:
* List x86_test-base in needs: along with x86_test-gl/vk (see parent
  commit)
* Always put $STABLE/TESTING_EPHEMERAL on separate lines, will make it
  easier to add any non-ephemeral packages

Reviewed-by: Eric Anholt <eric@anholt.net> # v1
Part-of: <mesa/mesa!5186>