The encoding issue is still there but after trying a lot of things I end up thinking that it is an issue with qemu-agent script. I tried as hard as I could to make powershell output utf-8, but it still doesn't seem to work and I suspect the output gets re-encoded to cp1152 somewhere in the Gitlab runner / qemu / virsh stack.
The issue comes from a kernel32:process test, which outputs CI_COMMIT_AUTHOR env var, which for me contains a é, later also output as a comma, which is encoded as a 0x82 comma (in Windows-1152) for some reason.
So I'm now simply making winetest output to a file, which is both read again to the console output in `after_script` step, and attached as an artifact to workaround cases where this error happens.
I also included a change to make the Windows tests only run the tests that a MR has modified, making it much faster to run.