Improved unittests detect different in arm64 rounding.

TEST=util/android/test_runner.py gtest -s libyuv_unittest -t 7200 --verbose --release --gtest_filter=*Half* -a "--libyuv_width=640 --libyuv_height=360"
BUG=libyuv:560
R=wangcheng@google.com

Review URL: https://codereview.chromium.org/2478313004 .