tests: reduce boilerplate of sending GET requests

This adds a `SendRequestExt` trait to h2-support, with a `get` method
that does a lot of the repeated request building stuff many test cases
were doing.

As a first step, the cleans up stream_states tests to use it.

tests/h2-support/src/prelude.rs

@@ -31,6 +31,9 @@ pub use self::futures::{Future, IntoFuture, Sink, Stream};
 // And our Future extensions
 pub use super::future_ext::{FutureExt, Unwrap};
 
+// Our client_ext helpers
+pub use super::client_ext::{SendRequestExt};
+
 // Re-export HTTP types
 pub use self::http::{uri, HeaderMap, Method, Request, Response, StatusCode, Version};