fix output file check in vpxenc tests script.
fix output file check in vpxenc tests script.

BUG=webm:1556

Change-Id: I4be40e9bf667cd9896017f38d866a47d3e19dcaf

ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx