Upgrade isotesters to Buster
Parent issue: tails#17842 (closed)
Having isotesters running Stretch, instead of Buster, blocks:
- tails#15831 (closed)
- tails#17031 (closed)
- 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'
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.