message: integration_tests: flush before aborting
  Otherwise we don't actually see the error message.
diff: @@ -18,6 +18,8 @@ protected:
  const int ret = system((std::string("./tools/start_px4_sitl.sh ") + model).c_str());
  if (ret != 0) {
  mavsdk::LogErr() << "./tools/start_px4_sitl.sh failed, giving up.";
  + fflush(stdout);
  + fflush(stderr);
  abort();
  }
  #else
  @@ -32,6 +34,8 @@ protected:
  const int ret = system("./tools/stop_px4_sitl.sh");
  if (ret != 0) {
  mavsdk::LogErr() << "./tools/stop_px4_sitl.sh failed, giving up.";
  + fflush(stdout);
  + fflush(stderr);
  abort();
  }
  #else
  