Improve test coverage for mp4
Improve test coverage for mp4

ossrs/srs,winlinvip/srs,ossrs/srs,ossrs/srs,winlinvip/srs,ossrs/srs,ossrs/srs,winlinvip/srs,winlinvip/simple-rtmp-server,winlinvip/srs,winlinvip/srs,winlinvip/srs,ossrs/srs