videoio(test): add FFmpeg backend check

- configure through OPENCV_TEST_VIDEOIO_BACKEND_REQUIRE_FFMPEG environment variable

Author: alalek

modules/videoio/test/test_main.cpp

@@ -2,5 +2,20 @@
 // 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())