message: libcurl: wait before checking file existence
  This will fix the timing issue in integration tests.
diff: #include "curl_wrapper.h"
  #include <fstream>
  #include <iostream>
  +#include <chrono>
  +#include <thread>
  using namespace dronelink;
  @@ -36,6 +38,7 @@ protected:
  bool check_file_exists(const std::string &file_path)
  {
  + std::this_thread::sleep_for(std::chrono::milliseconds(100));
  std::ifstream infile(file_path.c_str());
  return infile.good();
  }
  