videoio(ffmpeg): specify stream->time_base

Using codec->time_base is deprecated to specify muxer settings.
Resolves issue with FPS value for AVI files with FFmpeg 4.0.

Related FFmpeg commits:
- https://github.com/FFmpeg/FFmpeg/commit/194be1f43ea391eb986732707435176e579265aa
- https://github.com/FFmpeg/FFmpeg/commit/91736025b2807995e29bd0661807c1c84f515fef

modules/videoio/test/test_ffmpeg.cpp

@@ -357,6 +357,8 @@ public:
 
             for (unsigned int i = 0; i < frameCount && next; ++i)
             {
+                SCOPED_TRACE(cv::format("frame=%d", (int)frameCount));
+
                 Mat actual;
                 (*capture) >> actual;