add autopkgtest support

This is something Lintian complained about wrt v2019.02.04-1. I believe it has
now been upgraded to a blocker for packages migrating from Debian unstable to
Debian testing, hence the priority to address this.

To get this to work for v2019.09.15, we'll need to cherry pick
eca645c2 and play the usual tag juggling game.