libs.mk: retry partial testdata download
libs.mk: retry partial testdata download

attempt retry on transient failures uncaught by --retry

Change-Id: I7cd8846ff88daf0f521af9ee182e30bfd79f51f3

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