Add HTTP request tests

[brendanlong]

Right now our coverage of the actual network sending is lacking. If we had an HTTP server that returns mock data and status codes, we could do something like:

let project = 123 in
let mock_server = Mock_server.make ~path:"/api/123/store" ~return_status:200 in
let port = Mock_server.port mock_server in
let dsn = sprintf "http://key@localhost:%d/123" port in
Sentry.with_dsn dsn (fun () ->
  Sentry.send_capture_message "This is a test");
Sentry.wait_for_queue () (* would need to add this function *)
>>| fun () ->
Mock_server.get_request mock_server
|> [%test_result: Cohttp.Request.t] ~expect

[brendanlong]

Or maybe it would be easier to just break apart the function that creates HTTP requests and test that..