Add Buster support to the automated test suite

Originally created by @anonym on #17457 (Redmine)

I’ve already done some work on this on the test/15460-replace-sikuli branch, so I mostly want to create this ticket before any one else does and starts working on it.

I’ll use this ticket to list other issues I’ve noticed, but not solved yet.

• QEMU 4.2 backport for Buster (needed to support saving snapshots in a nested setup)
• Ability to run the test suite with --view from a Wayland desktop (#15460 (comment 22037))

And in passing, for Bullseye support, see #17031 (closed).

Related issues

• Related to #15953 (closed)
• Blocked by #15460 (closed)
• Blocks #15831 (closed)
• Blocks #15450