The sum of floats can optimize differently with vectorization, producing
a different result between NEON and C.
Adjust the unittest to allow for some difference in the sum.

The NEON version is 8 samples at a time, so the test now rounds up
the number of values to multiple of 8.

TBR=kjellander@chromium.org
Bug: libyuv:717
Test: LibYUVPlanarTest.TestScaleSumSamples_Opt


Change-Id: I2a0783780c7e0f240f7a8e4700b2a4d3e6b52d87
Reviewed-on: https://chromium-review.googlesource.com/673708Reviewed-by: Cheng Wang <wangcheng@google.com>