vp8: Remove duplicated code in datarate tests.
vp8: Remove duplicated code in datarate tests.

Duplicated code between *Large and other tests.

Change-Id: I0cea7472c3520175339bc921dfd8a090b5d5484d

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