Crosstesting

cross-building is starting to become mature enough so that we want to display cross-satisfiability results on pages like qa.debian.org/developer.php. For this to work, we need to produce results that are less flaky than those produced by testing unstable. The problem with unstable is, that regularly 1000+ source packages become uninstallable for a while because of M-A:same version skews. Testing is not as up-to-date as unstable but at least it won't annoy maintainers with false-negatives thanks to the guards set up by our migration to testing rules.