Reduce testing server result channel capacity to 1.

src/proto/server/testing.rs

@@ -177,9 +177,7 @@ impl ServerBuilder {
 
     let (shutdown_tx, shutdown_rx) = oneshot::channel();
     let (handler_shutdown_tx, handler_shutdown_rx) = watch::channel(());
-
-    // TODO: configurable channel capacity.
-    let (result_tx, result_rx) = mpsc::channel(100);
+    let (result_tx, result_rx) = mpsc::channel(1);
 
     Ok((
       Server {