What's up with test.thing (asg2025)

`test.thing` is a VM runner which targets guests using an API defined by systemd. It started after a conversation at devconf about turning `mkosi qemu` into a library. A quick intro. ~~composefs is an approach to image-mode systems without the disk images. Files are stored in a de-duplicated content-addressed storage with integrity guaranteed through fs-verity. The last year has seen an acceleration of development on composefs-rs, a pure Rust implementation of the ideas behind composefs. Our goal is unification of the storage of bootable system images (via bootc), application Flatpaks, and traditional OCI container environments, bringing deduplication and integrity guarantees to all three. An overview.~~ Licensed to the public under https://creativecommons.org/licenses/by/4.0/de/ about this event: https://cfp.all-systems-go.io/all-systems-go-2025/talk/MLTTHW/