Add tests

[radu-matei]

This repository should have a few simple integration tests to ensure the right files are served on the right routes with the right content types.

[ejmg]

I'll be taking a swing at this issue if nobody else is currently prioritizing it.

[ejmg]

Looking at this more, would an ultra-simplified version of spin's own integration tests be a good model to copy?

AFAICT, this library is meant to be used as a one off component, so testing behavior really just means building and running the component with spin and making test requests against it to check for the things mentioned above.