Method on_connection_open

cpp/src/reconnect_test.cpp:136–146

class tester_base: public proton::messaging_handler {
  void on_connection_open(proton::connection& c) override {
    std::string want_url = "amqp://localhost:" + std::to_string(listening_port_);

    if (!c.reconnected()) {
      start_count++;
      c.open_sender("messages");
    }
    ASSERT_EQUAL(c.url(), want_url);
    ASSERT_EQUAL(bool(open_count), c.reconnected());
    open_count++;
  }

  void on_connection_error(proton::connection& c) override {
    connection_error_count++;