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/lib.rs

@@ -22,8 +22,10 @@ pub mod mock_io;
 pub mod notify;
 pub mod util;
 
+mod client_ext;
 mod future_ext;
 
+pub use client_ext::{SendRequestExt};
 pub use future_ext::{FutureExt, Unwrap};
 
 pub type WindowSize = usize;