Set non-default values for the AC-3, E-AC-3 and fixed-point AC-3
decoder options, decode a frame so that the exported downmix fields
hold values set from the frame header, flush with
avcodec_flush_buffers(), and check that all values are still readable
afterwards. This is a regression test for the decoders forgetting
options such as drc_scale on flush.
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>