setup-testbed: prevent /tmp from getting a tmpfs mount

Newer systemd versions default to mounting a tmpfs on /tmp. This means the /tmp available size to half of the RAM installed on the system. Autopkgtest uses /tmp to unpack source trees and perform builds, that that sometimes takes multiple gigabytes.

This change disables the automatic tmpfs mount. We should probably switch to /var/log and keep the /tmp tmpfs (to stick with defaults as far as possible), but I believe autopkgtest makes assumptions on its scratch space to be cleared on reboots. This would not be the case when using /var/tmp, so the switch is not straightforward.

LP: #2069834