Upgrade isotesters to Buster

Parent issue: tails/tails#17842

Having isotesters running Stretch, instead of Buster, blocks:

• tails/tails#15450
• tails/tails#15831
• tails/tails#15330
• tails/tails#17031
• dropping some Stretch backwards compatibility code in our test suite

Additionally, this would allow me to stop maintaining a custom QEMU package for Stretch.

Setting milestone to 2020 as per the "Upgrade our systems to Buster" item in sysadmins' roadmap-2019Q3-2020Q2.mdwn.

One of the nodes (that does both builds & tests) in my local Jenkins setup runs Buster, and it works fine — actually, significantly faster than the Stretch one on the same hardware, which suggests this upgrade could be pretty much straightforward.