test: Avoid QEMU image overlays in /tmp on bots
test: Avoid QEMU image overlays in /tmp on bots

/tmp is a 10 GiB tmpfs on our bots. Our tests use too much disk space in
their VMs for that, causing regular overflows and VMs in the "paused"
state due to ENOSPC.

jkozol/welder-web,jkozol/welder-web,jkozol/welder-web,jkozol/welder-web