Macro assert_headers 

macro_rules! assert_headers {
    (@pair, $conn:expr_2021, $header_name:tt, None) => { ... };
    (@pair, $conn:expr_2021, $header_name:tt, $header_value:expr_2021) => { ... };
    ($conn:expr_2021, $($header_name:tt => $header_value:tt,)+) => { ... };
    ($conn:expr_2021, $($header_name:tt => $header_value:tt),*) => { ... };
}

asserts any number of response headers

use trillium_testing::prelude::*;
async fn handler(conn: Conn) -> Conn {
conn.ok("headers")
.with_response_header("server", "special-custom-server")
.with_response_header("request-id", "10")
}

assert_headers!(get("/").on(&handler), "server" => "special-custom-server");
assert_headers!(
get("/").on(&handler),
"server" => "special-custom-server",
"request-id" => "10",
"content-length" => "7"
);