Add tests

At the moment the tests are probably quite thorough because the hit a real API endpoint.
However this means they are somewhat fragile.

Also all the functions are marked as `TODO: test`
@lpil do you have an idea on how you would like to test these functions?

In magpie I ran tests against a locally running version of httpbin.org however that did add a dependence on docker, which I'm unsure if you want to add as a requirement for contributing. I also haven't played much with github workflows so am unsure what that would take

https://github.com/midas-framework/magpie/blob/master/test/magpie_test.gleam#L12
https://github.com/midas-framework/magpie/blob/master/docker-compose.yml#L24-L27