Move tests and support utilities to sub crates. (#268)

These crates will not be published to crates.io, but moving them allows
`tower-h2` to also depend on the test utilities.

tests/h2-support/src/util.rs

@@ -0,0 +1,44 @@
+use h2;
+
+use super::string::{String, TryFrom};
+use bytes::Bytes;
+use futures::{Async, Future, Poll};
+
+pub fn byte_str(s: &str) -> String<Bytes> {
+    String::try_from(Bytes::from(s)).unwrap()
+}
+
+pub fn wait_for_capacity(stream: h2::SendStream<Bytes>, target: usize) -> WaitForCapacity {
+    WaitForCapacity {
+        stream: Some(stream),
+        target: target,
+    }
+}
+
+pub struct WaitForCapacity {
+    stream: Option<h2::SendStream<Bytes>>,
+    target: usize,
+}
+
+impl WaitForCapacity {
+    fn stream(&mut self) -> &mut h2::SendStream<Bytes> {
+        self.stream.as_mut().unwrap()
+    }
+}
+
+impl Future for WaitForCapacity {
+    type Item = h2::SendStream<Bytes>;
+    type Error = ();
+
+    fn poll(&mut self) -> Poll<Self::Item, ()> {
+        let _ = try_ready!(self.stream().poll_capacity().map_err(|_| panic!()));
+
+        let act = self.stream().capacity();
+
+        if act >= self.target {
+            return Ok(self.stream.take().unwrap().into());
+        }
+
+        Ok(Async::NotReady)
+    }
+}