The issue is that the /projects path is no longer served by the server. It's now /api/read/file/ followed by a urlencoded path. For instance, for image img.jpg in project PROJECT, it is PROJECT%2Fimg.jpg.
In other words, in your case it will be:
img = loadImage('/api/read/file/test%2Ftest.png');
If at any point you have a doubt, just open the image in the IDE and right click on it and select "copy image address" to get the URL, which will look - in this case - like: http://bela.local/api/read/file/PROJECT%2Fimg.jpg?t=1773261628203.0884 . You can remove the leading http://bela.local and the trailing ?t=....
I need to see if we can safely re-enable the old path for backwards compatibilty.