Read file from tests

Hello!

It would be really awesome if we were able to read files from our elm tests! As an example, consider canned JSON responses from an API. It would be great to keep these as separate `.json` files instead of bringing them into my elm source files as multi-line strings.

See this related Stack Overflow post: https://stackoverflow.com/questions/48378601/load-external-data-into-elm-test-suite

The solution in the post proposed using Native Modules. Unfortunately, because native modules have been (mostly) deprecated as of 0.19, it seems that the only way to reasonably use this approach to read files would be through `elm-explorations/test`. I think this a really reasonable use case for testing and would like to propose it as a feature request. I'm happy to help propose what an API might look like and implement a prototype if there is support from the admins.

Thanks,
Advait