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/future_ext.rs

@@ -72,20 +72,11 @@ pub trait FutureExt: Future {
     ///
     /// This allows the executor to poll other futures before trying this one
     /// again.
-    fn yield_once(self) -> Box<Future<Item = Self::Item, Error = Self::Error>>
+    fn yield_once(self) -> Box<dyn Future<Item = Self::Item, Error = Self::Error>>
     where
         Self: Future + Sized + 'static,
     {
-        let mut ready = false;
-        Box::new(::futures::future::poll_fn(move || {
-            if ready {
-                Ok::<_, ()>(().into())
-            } else {
-                ready = true;
-                ::futures::task::current().notify();
-                Ok(::futures::Async::NotReady)
-            }
-        }).then(|_| self))
+        Box::new(super::util::yield_once().then(move |_| self))
     }
 }