This is one of the places where you’ll need to differentiate between doing an http requests and using plug/phoenix test helpers for creating requests.
Just because your requests blows up here – and it’s supposed to blow up – doesn’t mean your client will receive no response. send_resp/3 in your error handler will make sure the client to the http request is being sent a response. That the process for the web requests blows up afterwards is totally irrelevant. Cowboy will catch that, close the connection and be fine.
For tests you want the error to not be swallowed, so you can catch what’s going wrong. You can also assert_raise if you expect an error. If you want to assert that a client for the http request is not affected you’ll need to do a proper http request using something like httpoison or similar (also make sure to enable the server for the test env, server: true). Basically the misconception here it that conn(conn, "/products", body_params) is doing an http request. It more or less calls MyApp.Endpoint.call(conn, …), even more bare bone for plain plug. No cowboy involved in the slightest. So if you want to actually assert the error handling, which involves more than just the plug parts, you need to fall back to a more involved testing method.
