Draft: test suite can be run without the .img file

this is useful when you're downloading images from Jenkins: downloading the ISO is functionally enough, but you need the IMG file, just to make our test suite happy. This MR makes this check lighter, only providing a warning if the .img file doesn't exist.

Yes, there are some testcases that actually do need the .img file: we hope the developer knows, or they will face an error :)