Step "wget(1) with tricky options" broken as it relies on outdated 220.127.116.11 IP
It was moved to 18.104.22.168. Affects stable, devel and feature/stretch.
Resolve dl.amnesia.boum.org instead of picking a static address.
Just hours after updating the dustri.org IP address, its web server went
down => test suite failures. Let's make this test as robust as actually
downloading the Tails ISO image -- if that fails, we probably have more
serious problems on our hands than a failing test suite.