This should fix some tests, as a lot of them are dependent on
deb.debian.org which no longer supports Debian 9 "Stretch".
Instead we use archive.debian.org which will continue to contain
"Stretch" packages for a long time.
Use CDN-backed Debian mirror to make tests run faster hopefully for
everyone. Redirects might be important to know what exactly is going on
when items are being downloaded.
Mauro Regli
Lorenzo Bolla
Andrey Smirnov