Document/add fixture for extra model downloads

[ElliottKasoar]

I know this isn't newly introduced, but I just realised this isn't run as part of the tests, but only in the CI? Ideally we'd probably encourage devs to run this and/or include downloading them (optionally) locally?

Otherwise it might be confusing to run the tests fine locally and then things fail in the CI?

Originally posted by @ElliottKasoar in #662