Introduces an intermediate format for overlayfs storage, where
.wh-ostree. prefixed files will be converted into char 0:0
whiteout devices used by overlayfs to mark deletions across layers.
The CI scripts now uses a volume for the scratch directories
previously in /var/tmp otherwise we cannot create whiteout
devices into an overlayfs mounted filesystem.
Related-Issue: #2712
`kola` now follows symlinks when archiving an external test's `data/`
dir. So the recursive `data` symlink we have here breaks it.
Let's just move the shared files in its own directory and update the
symlinks.
Miguel Angel Ajo
Jonathan Lebon