Upgrade isotesters to Buster
Parent issue: tails#17842
Having isotesters running Stretch, instead of Buster, blocks:
- dropping some Stretch backwards compatibility code in our test suite
Additionally, this would allow me to maintain 1 custom QEMU package (Buster) instead of 2 (Stretch and Buster): the current situation, introduced by tails#15460 (closed), was meant to be very temporary, as per the discussion on tails#17457 (comment 2437), where @anonym and I agreed we should notify sysadmins ASAP about our need for this upgrade; but apparently we closed the issue without communicating about this :/
Setting milestone to 2020 as per the "Upgrade our systems to Buster" item in sysadmins'