test: limit the valid image size on OS/2
test: limit the valid image size on OS/2

Without this, test_libvpx crashes at VP9FrameSizeTestsLarge.ValidSizes
on OS/2.

Change-Id: If253fb95a04216babed56458f0fab8c70523fc7e

smarter/aom,jmvalin/aom,GrokImageCompression/aom,openpeer/libvpx_new,mbebenita/aom,luctrudeau/aom,shacklettbp/aom,mwgoldsmith/vpx,GrokImageCompression/aom,VTCSecureLLC/libvpx,webmproject/libvpx,ittiamvpx/libvpx-1,mbebenita/aom,luctrudeau/aom,webmproject/libvpx,mbebenita/aom,ShiftMediaProject/libvpx,thdav/aom,shacklettbp/aom,ittiamvpx/libvpx-1,mbebenita/aom,openpeer/libvpx_new,Topopiccione/libvpx,mwgoldsmith/libvpx,openpeer/libvpx_new,ittiamvpx/libvpx-1,ShiftMediaProject/libvpx,Topopiccione/libvpx,ittiamvpx/libvpx-1,mbebenita/aom,mwgoldsmith/libvpx,webmproject/libvpx,mwgoldsmith/vpx,jmvalin/aom,thdav/aom,webmproject/libvpx,thdav/aom,shacklettbp/aom,thdav/aom,GrokImageCompression/aom,Topopiccione/libvpx,VTCSecureLLC/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,webmproject/libvpx,smarter/aom,VTCSecureLLC/libvpx,ittiamvpx/libvpx-1,ShiftMediaProject/libvpx,mwgoldsmith/vpx,felipebetancur/libvpx,shacklettbp/aom,jmvalin/aom,felipebetancur/libvpx,Topopiccione/libvpx,felipebetancur/libvpx,mbebenita/aom,thdav/aom,luctrudeau/aom,luctrudeau/aom,smarter/aom,webmproject/libvpx,smarter/aom,felipebetancur/libvpx,GrokImageCompression/aom,smarter/aom,mbebenita/aom,mwgoldsmith/vpx,GrokImageCompression/aom,openpeer/libvpx_new,openpeer/libvpx_new,Topopiccione/libvpx,mwgoldsmith/vpx,shacklettbp/aom,felipebetancur/libvpx,jmvalin/aom,luctrudeau/aom,jmvalin/aom,mwgoldsmith/libvpx,VTCSecureLLC/libvpx,VTCSecureLLC/libvpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,thdav/aom,VTCSecureLLC/libvpx,Topopiccione/libvpx,felipebetancur/libvpx,jmvalin/aom,luctrudeau/aom,openpeer/libvpx_new,shacklettbp/aom,mwgoldsmith/libvpx,GrokImageCompression/aom,mbebenita/aom,mbebenita/aom,smarter/aom,ittiamvpx/libvpx-1