tests that use the loopback network interface fail under ci-image-git-buildpackage

I tried setting up ci-image-git-buildpackage for (a local version of) tinc, and the tests fail, seemingly due to not having a loopback network interface.

For reference, since version 4.2.0, the Debian policy state in 4.9 that:

For packages in the main archive, no required targets may attempt network access, except, via the loopback interface, to services on the build host that have been started by the build.

Originally filed at https://gitlab.com/eighthave/ci-image-git-buildpackage/issues/2 by @nicoonoclaste