Use the fast fast-single-thread runner when using qemu for testing

The fast-single-thread runner is much faster (test-cloudimg-qemu):
3min 39sec[1] vs 8min 37sec[2]

[1] https://gitlab.archlinux.org/klausenbusk/arch-boxes/-/jobs/14739
[2] https://gitlab.archlinux.org/klausenbusk/arch-boxes/-/jobs/14746
6 jobs for fast-single-thread-test in 26 minutes and 56 seconds (queued for 1 second)