Create a test framework that runs as a default-unprivileged container

Spawned from https://github.com/containers/bootc/pull/1146#discussion_r1972360016

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I bet though we could (and really should) introduce a test framework that runs as a default-unprivileged container (i.e. uid0, where we have full capability to invoke setuid).

Although it looks like

$ podman run --rm -ti localhost/fedora-bootc bash
bash-5.2# sshd -T
sshd: no hostkeys available -- exiting.
So we'd have to do a bit of work to set that up