add unit test, use md5 and psnr to test the validity of decode.

usage:
./unit_test.sh  /root/Videos/h264/Base_Ext_Main_profile/
./unit_test.sh  /root/Videos/vp8/vp8/
./unit_test.sh  /root/Videos/jpg/workdir.jpeg_dec/ /root/Videos/jpg/yuv/
refer the script in yami wiki, not gst-omx wiki (it is too complex than needed)