Add connection reset tests to cohttp-eio

[mseri]

It might be nice to add a test for connection reset errors. The old cohttp-eio had some tests where you could inject errors (though it only tried with End_of_file), but it looks like they got removed in the rewrite: https://github.com/mirage/ocaml-cohttp/blob/b7182a17e9769c7d08c5b05e2512e0eca605f74e/cohttp-eio/tests/server.md#tests

From #1015