Actually test the fetcher in nixos tests #268
Description
Our nixos tests run pr-tracker-fetcher, but it doesn't actually do anything (because it cannot talk to github). As a result, we had this bug when deploying to a real server: #269
We do unittest test the pr-tracker-fetcher code with some clever rust trait shenanigans. IMO, this both complicates the code and doesn't do us any good for our nixos tests. I believe it would be better to implement a simple mocked graphql service instead.