Tests for the `wm-convert` binary

What do you think about adding a couple of (integration-) tests for running the binary? I guess we could have a single small image in the repo to test * one invocation of `-resize` * one invocation of `-identify` Obviously we'd not enumerate like all of the options and also not check in 100s of MB of images. But there's a bit of risk breaking the overall integration of things when just concentrating on testing the fiddly algorithmic parts, right? As it stands right now the coverage of some quite integral parts are minimal, and the `plan.rs` and so on are probably kind of tricky to write tests for inside the file? <img width="1465" height="834" alt="Image" src="https://github.com/user-attachments/assets/d6b08cdc-3d14-418f-a156-b53c3dcf5fc9" />