v2021 ← v2023dev3: tests: Check that KVM is now the default

The VM type used by the default GitLab runners has been changed to one that provides nested vistualization support and the runner configuration has been tweaked to expose it to containers.

The VM type used by the default GitLab runners has been changed to one that provides nested vistualization support and the runner configuration has been tweaked to expose it to containers.

Now both the KVM and UML Debos backends can be used on the runners, and Debos will prefer the KVM as it yields a nice performance speedup.

This means that the test checking the availability of the UML backend fails since the KVM is picked up automatically. Explicitly setting the desired backend avoids the reliance on autodetection.

By not forcing the backend we can check that the Debos autodetection now picks up KVM since it is faster.

Backport of !264 (merged)