adt tests failing for clamav (./unit_tests/accdenied)

Hi,
as discussed on IRC here a summary to track.

I have found that the clamav dep8 tests as they are in the archive don't work when ran in a local kvm based adt environment.

Reproduction with:
cd /; rm -rf /tmp/test-adt-clamav; mkdir -p /tmp/test-adt-clamav; cd /tmp/test-adt-clamav; pull-lp-source clamav; adt-run --shell-fail --apt-upgrade --source clamav_0.99+dfsg-1ubuntu2.dsc --- adt-virt-qemu --cpus 4 --ram-size=2048 ~/work/adt-yakkety-amd64-cloud.img

Now it seems that the build itself is part of the reason as pitti found that with -B it works fine.

Also I found that running with adt-virt-lxd triggered the same bug, but after adding -ddd to adt-run and -d to adt-virt-lxd it went away.
After it worked once it always worked with lxd based autopkgtest even without debug flags.
But the kvm based still failed with the same error.