zfstest is trying to execute evil "zfs unmount -a", which fails (fortunately,
as it would otherwise leave me with my ~ missing):
03:44:11.86 cannot unmount '/export/home/yuri': Device busy cannot unmount '/
export/home': Device busy
03:44:11.86 ERROR: /usr/sbin/zfs unmount -a exited 1
This affects, at least, zfs_mount_009_neg and zfs_mount_all_001_pos, both
failing on that step. The pool containing the /export/home hierarchy is
included in KEEP variable, but it doesn't seem to affect anything here.