How do we add image tests?

[rcomer]

During the course of #4792, I realised I have no idea how to add an image test to Iris. Changing the reference image is well documented and idiff.py sorts out the hash for the image name and json file entry. If adding a new reference image, do you just do this manually or is there a tool? Is there some documentation I've missed?

I thought about opening this as a documentation issue but

• we don't add new image tests very often
• we may be changing image testing approach soon

so documenting it right now may be a waste of time.

[rcomer]

I see @wjbenfold already has this covered in #4759 👀

[wjbenfold]

I've also struggled to work this out previously - it'd be good to know if you think the docs explanation in #4759 make sense :)

[rcomer]

Now documented at https://scitools-iris.readthedocs.io/en/latest/developers_guide/contributing_graphics_tests.html#adding-a-new-image-test.