test: limit the valid image size on OS/2

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

Change-Id: If253fb95a04216babed56458f0fab8c70523fc7e
diff --git a/test/frame_size_tests.cc b/test/frame_size_tests.cc
index 95cc66a..d39c8f6 100644
--- a/test/frame_size_tests.cc
+++ b/test/frame_size_tests.cc
@@ -74,7 +74,7 @@
   // size or almost 1 gig of memory.
   // In total the allocations will exceed 2GiB which may cause a failure with
   // mingw + wine, use a smaller size in that case.
-#if defined(_WIN32) && !defined(_WIN64)
+#if defined(_WIN32) && !defined(_WIN64) || defined(__OS2__)
   video.SetSize(4096, 3072);
 #else
   video.SetSize(4096, 4096);