message: Set manual multiplexing in testing MM backend
diff: @@ -168,7 +168,7 @@ expected<uint16_t> middleman::publish_local_groups(uint16_t port,
  }
  };
  };
  - auto gn = system().spawn<hidden>(group_nameserver);
  + auto gn = system().spawn<hidden + lazy_init>(group_nameserver);
  auto result = publish(gn, port, in, reuse);
  // link gn to our manager
  if (result)
  @@ -357,8 +357,10 @@ void middleman::init(actor_system_config& cfg) {
  // never detach actors when using the testing multiplexer
  auto network_backend = get_or(cfg, "middleman.network-backend",
  defaults::middleman::network_backend);
  - if (network_backend == atom("testing"))
  - cfg.set("middleman.attach-utility-actors", true);
  + if (network_backend == atom("testing")) {
  + cfg.set("middleman.attach-utility-actors", true)
  + .set("middleman.manual-multiplexing", true);
  + }
  // add remote group module to config
  struct remote_groups : group_module {
  public:
  