introduce an extension trait for testing servers

[jbr]

This is a first step in the direction of a superagent/supertest type testing utility for tide. Currently it just supports simple requests and is not exported for usage outside of tide, but the intent is that as it accumulates more features and stabilizes, it might eventually be public under a tide::testing namespace for tide apps to use.

// this:
    let req = Request::new(
        Method::Get,
        Url::parse("http://example.com/foo/echo").unwrap(),
    );
    let mut res: Response = app.respond(req).await.unwrap();

// becomes:
    let mut res = app.get("/foo/echo").await;

and

 // this:
    let req = http::Request::new(
        Method::Get,
        Url::parse("http://localhost/add_one/3").unwrap(),
    );
    let mut res: http::Response = app.respond(req).await.unwrap();
    assert_eq!(&res.body_string().await.unwrap(), "4");

// becomes:
    assert_eq!(app.get_body("/add_one/3").await, "4");

[Fishrock123]

This change generally seems nice for tests, maybe there's some detail I'm not able to think of but otherwise lgtm.

[yoshuawuyts]

LGTM!

[Fishrock123]

still lgtm