Rewrite `.build-empty-images` shell script that produced special images
(emptyfs with no layers, and empty danglign image) to a Go functions
that construct the same archives in a temporary directory.
Use them to load these images on demand only in the tests that need
them.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
bc94dfc7d2