selftests/landlock: Make mounts configurable

Add a new struct mnt_opt to define a mount point with the mount_opt()
helper.  This doesn't change tests but prepare for the next commit.

Link: https://lore.kernel.org/r/20230612191430.339153-5-mic@digikod.net


Signed-off-by: Mickaël Salaün <mic@digikod.net>