Define and ship a "testutils" container

[cgwalters]

In several of ostree/rpm-ostree external tests we fetch a container image to run privileged for helper utilities, whether that's python or a webserver program.

This somewhat commonly causes tests to flake; particularly the Fedora registry seems unreliable, but docker.io also has rate limited us in the past. And most of these tests don't otherwise need the internet.

I'd like if as part of coreos-assembler we defined a "test helper" container (perhaps this should be an official FCOS toolbox container) that contains things like a webserver, strace, etc.

Now what we could do in the qemu-unpriv case is be able to pull this container content over virtio (fs or channel) from the "host" container image and avoid the dependency on networking.

In the non-qemu-unpriv case we'd host this image at a public registry somewhere. But if we eventually wanted to support "offline" tests even in cloud, we'd need to provision a mirror of testutils in the environment.

[dustymabe]

If we did this maybe we could roll the test/functionality from podman.workflow into it and we could stop denylisting it (#1478).

[dustymabe]

This overlaps a bit with the newly created tracker issue coreos/fedora-coreos-tracker#1639 that @jbtrystram is working on. I'll close this out and we'll track it there.