Tests: add VSE render test coverage for ffmpeg movie decoding

Specifically coverage of various YUV formats (4:2:0 and 4:4:4),
some bit depths (most 8 bit, some 10 bit), various colorspace
settings (untagged, bt709), and YUV color ranges (regular/limited vs
full/pc).

build_test_movies.sh in there generates all the (tiny) movie files
out of input color_chart.png using command line ffmpeg.

tests/python/CMakeLists.txt

@@ -1113,6 +1113,7 @@ else()
     transform
     blend_modes_byte
     blend_modes_float
+    ffmpeg
   )
 
   foreach(render_test ${render_tests})