Refactor and document testing

[cthoyt]

The /test directory is very hard to remember how it works. This PR does some minor refactoring and adds a README to make it more clear how testing works

1. Puts all of the expected-paper.* into a new folder expected-draft/
2. Updates the Makefile accordingly
3. Adds a README

[cthoyt]

@xuanxu do you have any idea why these tests might be failing? I also experienced in a different branch that the make test command inside the image builder workflow fails. Is it possible there is some metadata injected by virtue of it being a PR that makes it fail?

[tarleb]

Thanks that's a significant improvement. The tests were just hacked together to catch the worst problems, a cleanup is most welcome.

I'm sorry for being so slow right now, there are too many things that I want to do simultaneously 🙈

Merging this despite the failing tests, as we're planning to remove the "golden pdf" tests anyway.