Most conversion tests exactly match on Intel, so
reduce max diff to zero for Intel and leave at 4 for
ARM using a macro.

TBR=braveyao@chromium.org
Bug: libyuv:447
Test: libyuv_unittests pass
Change-Id: Ife13a0dd9581b58135b39d95ff1081f74840655c
Reviewed-on: https://chromium-review.googlesource.com/832927Reviewed-by: Frank Barchard <fbarchard@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>