Test Commits in server, test `client`

Commits are not tested in the server, even though there are quite a bit of things that could go wrong. They are tested in the end-to-end tests, though, but these are very slow. The problem with testing the the front-end, is that I'm not using atomic_lib:client, which would make things easier. However, testing with actix means using its test::TestRequest struct, which means that I have to re-implement requests / client. I don't like that. But... re-implementing these isn't too much work. We can set the method like this:

    let req = build_request_authenticated("/commit", &appstate).method(Method::POST);

Start by comparing the existing end-to-end tests for commits with the server code handling POST requests to /commit. Review how actix::test::TestRequest and build_request_authenticated can represent the client requests without atomic_lib::client. Done means server-side commit tests cover the relevant request behavior without relying on the slower end-to-end suite.