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These crates will not be published to crates.io, but moving them allows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/raw.rs

@@ -0,0 +1,52 @@
+// ===== Build a codec from raw bytes =====
+
+#[macro_export]
+macro_rules! raw_codec {
+    (
+        $(
+            $fn:ident => [$($chunk:expr,)+];
+        )*
+    ) => {{
+        let mut b = $crate::mock_io::Builder::new();
+
+        $({
+            let mut chunk = vec![];
+
+            $(
+                $crate::raw::Chunk::push(&$chunk, &mut chunk);
+            )+
+
+            b.$fn(&chunk[..]);
+        })*
+
+        $crate::Codec::new(b.build())
+    }}
+}
+
+pub trait Chunk {
+    fn push(&self, dst: &mut Vec<u8>);
+}
+
+impl Chunk for u8 {
+    fn push(&self, dst: &mut Vec<u8>) {
+        dst.push(*self);
+    }
+}
+
+impl<'a> Chunk for &'a [u8] {
+    fn push(&self, dst: &mut Vec<u8>) {
+        dst.extend(*self)
+    }
+}
+
+impl<'a> Chunk for &'a str {
+    fn push(&self, dst: &mut Vec<u8>) {
+        dst.extend(self.as_bytes())
+    }
+}
+
+impl Chunk for Vec<u8> {
+    fn push(&self, dst: &mut Vec<u8>) {
+        dst.extend(self.iter())
+    }
+}