vc1testenc: give muxer same name as demuxer

vc1testenc: give muxer same name as demuxer

Otherwise ffmpeg -formats claims that we only support demuxing
of that format.
To keep compatibility the struct could be duplicated instead,
but this seems almost like overkill for such a rare format.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>