Create API tests for image endpoints

[Nutomic]

Requirements

• Is this a feature request? For questions or discussions use https://lemmy.ml/c/lemmy_support
• Did you check to see if this issue already exists?
• Is this only a feature request? Do not put multiple feature requests in one issue.
• Is this a backend issue? Use the lemmy-ui repo for UI / frontend issues.

Is your proposal related to a problem?

At the moment we dont have any API tests for image uploads and related functionality. This would be very helpful to ensure that functionality such as image proxying (#4035) is working correctly.

Describe the solution you'd like.

Make pictrs available for API tests and write test cases for basic functionality like uploading and downloading an image. There are various ways to run pictrs, including binary downloads from the project repo. That is probably the easiest option, but it still requires some additional system dependencies like ffmpeg.

Describe alternatives you've considered.

Manual testing like we are doing now. Not a great plan.

Additional context

No response