Method on_container_start

cpp/src/context_test.cpp:104–107

    test_client (const std::string &s) : url(s) {}

    void on_container_start(proton::container &c) override {
        proton::connection_options co;
        sender = c.open_sender(url, co);
    }

    void on_connection_open(proton::connection& c) override {
        // Get default session