tests: the use of ubuntu-16.04 container images limits test runner host to cgroupv1

Many of the test definitions currently declare Ubuntu 16.04 container images for running their tasks.

This is a problem, because running a Ubuntu 16.04 container requires the host system to run cgroupv1, effectively preventing successful test runs on anything more recent than Ubuntu 20.04 (Focal Fossa).

If there is a need to keep running tasks on Ubuntu 16.04 we ought to use VM systems.

If there is no need, we ought to switch to newer images.

Inspect the test definitions that declare Ubuntu 16.04 container images and determine whether any tasks still require that environment. Run the affected test tasks on a newer host or image; done means the definitions no longer require cgroupv1, or the need for VM-based Ubuntu 16.04 execution is documented.

Written by the indexing model from the issue text.