debian/tests/control: run debootstrap with unshared namespaces

Thank you for merging !91 (merged) which fixed the problem described in bug #1031222 where systemd-tmpfiles exited early because /proc wasn't mounted. Mounting /proc using mount -o rbind /proc "$TARGET/proc" as implemented as part of !91 (merged) indeed fixes this problem but it creates another problem. When that code path is taken (for example when running autopkgtest on salsaci) /proc will not get unmounted after debootstrap finished. To fix this for good I extended the debootstrap autopkgtest to also test running debootstrap in an unshared user namespace.

The reason I'm marking this MR as "Draft" is that I'm not happy with using umount --lazy as the solution to the problem. I much rather would use umount --recursive but that fails with

umount: [...]/proc/sys/net: not mounted

Even if /proc is bind-mounted with rslave as a mount option. If on the other hand one mounts /proc with bind instead of using rbind then unmounting is no problem but as the added autopkgtest shows, mounting /proc that way will still let systemd-tmpfiles exit early without creating its files.

Does anybody have a better idea or is umounting with --lazy the way to go?