We should run all of --suite=mariabackup tests with at least two different combinations for innodb_undo_tablespaces. The default value is 0.
A major problem is that the restore tests always invoke --copy-back without an innodb_undo_tablespaces parameter. I tested both by invoking
and by creating an .opt file with the parameter in it.
Backup does get the parameter right, maybe also prepare does get it.