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/client_ext.rs

@@ -0,0 +1,30 @@
+use bytes::IntoBuf;
+use http::Request;
+use h2::client::{ResponseFuture, SendRequest};
+
+/// Extend the `h2::client::SendRequest` type with convenience methods.
+pub trait SendRequestExt {
+    /// Convenience method to send a GET request and ignore the SendStream
+    /// (since GETs don't need to send a body).
+    fn get(&mut self, uri: &str) -> ResponseFuture;
+}
+
+impl<B> SendRequestExt for SendRequest<B>
+where
+    B: IntoBuf,
+    B::Buf: 'static,
+{
+    fn get(&mut self, uri: &str) -> ResponseFuture {
+        let req = Request::builder()
+            // method is GET by default
+            .uri(uri)
+            .body(())
+            .expect("valid uri");
+
+        let (fut, _tx) = self
+            .send_request(req, /*eos =*/true)
+            .expect("send_request");
+
+        fut
+    }
+}