Add fragmented mp4 test

The new test file
video_480x360_mp4_h264_1350kbps_30fps_aac_stereo_128kbps_44100hz_fragmented
was generated from the existing
video_480x360_mp4_h264_1350kbps_30fps_aac_stereo_128kbps_44100hz
with ffmpeg:
$ ffmpeg -re \
 -i video_480x360_mp4_h264_1350kbps_30fps_aac_stereo_128kbps_44100hz.mp4 \
 -f mp4 -movflags frag_keyframe+empty_moov -codec copy \
 video_480x360_mp4_h264_1350kbps_30fps_aac_stereo_128kbps_44100hz_fragmented.mp4
and contains the same data, just fragmented. It has no sidx box, since ffmpeg
does not appear to be capable of generating those.

Change-Id: I81ed29241e6ef91dd574447182e04c6b359b3e8a