message: videoio(test): add FFmpeg backend check
  configure through OPENCV_TEST_VIDEOIO_BACKEND_REQUIRE_FFMPEG environment variable
diff: // It is subject to the license terms in the LICENSE file found in the top-level directory
  // of this distribution and at http://opencv.org/license.html.
  #include "test_precomp.hpp"
  +#include <opencv2/core/utils/logger.hpp>
  -CV_TEST_MAIN("highgui")
  +static
  +void initTests()
  +{
  +#ifndef WINRT // missing getenv
  + const std::vector<cv::VideoCaptureAPIs> backends = cv::videoio_registry::getStreamBackends();
  + const char* requireFFmpeg = getenv("OPENCV_TEST_VIDEOIO_BACKEND_REQUIRE_FFMPEG");
  + if (requireFFmpeg && !isBackendAvailable(cv::CAP_FFMPEG, backends))
  + {
  + CV_LOG_FATAL(NULL, "OpenCV-Test: required FFmpeg backend is not available (broken plugin?). STOP.");
  + exit(1);
  + }
  +#endif
  +}
  +
  +CV_TEST_MAIN("highgui", initTests())
  