Add a local backend for the tests

This is a follow-up of #94, it will add offline capabilities to the test suite.

All tests pass consistently except `ban.sh` which still sometimes fails on my x86_64 machine. Starting `varnishd` with `ban_lurker_sleep=0` doesn't help at all, so I suspect Varnish to be the culprit.

This build for Fedora illustrates both the purpose of this patch (as it works without access to @KristianLyng's blog =) and the inconsistent failure of `ban.sh`:
http://koji.fedoraproject.org/koji/taskinfo?taskID=6267552

_The failing build is the arm one, which I believe not to be supported by you, but that will be my responsibility as a maintainer to help it work._

One more thing, this patch uses `seq` which doesn't exist on FreeBSD IIRC. Since I don't run FreeBSD anywhere yet, I leave this fix to you. _/kthxbye_