Add CORS tests

[Nadrieril]

This is a draft to let me figure out how to do this
EDIT: no longer a draft. This PR works as far as I can tell. The PR is self-referential cause I use raw.githubusercontent as target for some of the tests, but merging doesn't change the merged commits so this should keep working.

[Nadrieril]

I'm confused: with latest dhall-haskell (1.38.1), if I do

echo "https://raw.githubusercontent.com/Nadrieril/dhall-lang/cors-tests/tests/import/data/cors/AllowedAll.dhall" | dhall

I get a "Not CORS compliant" error. But this fetches https://test.dhall-lang.org/cors/AllowedAll.dhall which returns a "*" CORS header.

curl -i -X OPTIONS https://test.dhall-lang.org/cors/AllowedAll.dhall

returns, as expected, access-control-allow-origin: *.

I don't see what could be wrong, @Gabriel439 do you know what might be happening? Could this be an issue with dhall-haskell?
EDIT: I did some tests with the rust impl and managed to get something working. I think it's a dhall-haskell bug, unless my impl is too lenient somewhere

[Gabriella439]

Yeah, that seems like a bug in dhall-haskell. I'm looking into it.

[Nadrieril]

Ok, then I'll trust my own implementation. This PR works as far as I can tell. The PR is self-referential cause I use raw.githubusercontent as target for some of the tests, but merging doesn't change the merged commits so this should keep working.