Support overlayfs whiteouts on checkout

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

tests/kolainst/data-shared/libtest-core.sh

@@ -163,6 +163,13 @@ assert_file_has_mode () {
     fi
 }
 
+assert_is_whiteout_device () {
+    device_details="$(stat -c '%F %t:%T' $1)"
+    if [ "$device_details" != "character special file 0:0" ]; then
+        fatal "File '$1' is not a whiteout character device 0:0"
+    fi
+}
+
 assert_symlink_has_content () {
     if ! test -L "$1"; then
         fatal "File '$1' is not a symbolic link"