test: re-export macro dependencies (#1077)

Callers may not always have `futures` available at the root of the
crate. Re-exporting dependencies makes them available to the macro at a
deterministic location.

tokio-test/src/lib.rs

@@ -21,3 +21,10 @@ extern crate tokio_timer;
 pub mod clock;
 mod macros;
 pub mod task;
+
+#[doc(hidden)]
+pub mod codegen {
+    pub mod futures {
+        pub use futures::*;
+    }
+}