fix inconsistent test server behavior

Co-authored-by: Brian Quinlan <brian@sweetapp.com>

pkgs/http_client_conformance_tests/lib/src/request_cookies_server.dart

@@ -25,16 +25,17 @@ void hybridMain(StreamChannel<Object?> channel) async {
       final request = utf8.decoder.bind(socket).transform(const LineSplitter());
 
       final cookies = <String>[];
-      request.listen((line) {
+      await for (final line in request) {
         if (line.toLowerCase().startsWith('cookie:')) {
           cookies.add(line);
         }
 
         if (line.isEmpty) {
           // A blank line indicates the end of the headers.
           channel.sink.add(cookies);
+          break;
         }
-      });
+      }
 
       socket.writeAll(
         [