/mnt/test is not unmounted at the end of the successful grml-debootstrap run.

root of all evil are the numerous running daemons after installation:
authdaemond
courierd
courierdsn
courieresmtp
courierfax
courierfilter
courierlocal
courierlogger
couriertcpd
courieruucp
cron
famd
portmap