apacheGH-41780: [C++][Flight][Benchmark] Ensure waiting server ready

We should read from result stream to get an error of this RPC. If we
don't read from result stream, we can't detect an error of this RPC.

cpp/src/arrow/flight/flight_benchmark.cc

@@ -131,7 +131,8 @@ struct PerformanceStats {
 Status WaitForReady(FlightClient* client, const FlightCallOptions& call_options) {
   Action action{"ping", nullptr};
   for (int attempt = 0; attempt < 10; attempt++) {
-    if (client->DoAction(call_options, action).ok()) {
+    auto result_stream_result = client->DoAction(call_options, action);
+    if (result_stream_result.ok() && (*result_stream_result)->Drain().ok()) {
       return Status::OK();
     }
     std::this_thread::sleep_for(std::chrono::milliseconds(1000));