Add test for SplitUVPlane and MergeUVPlane

Add public methods SplitUVPlanes and MergeUVPlanes based on the
optimized assembly functions that already exists.

TEST=SplitUVPlane unittest
BUG=libyuv:629
R=braveyao@chromium.org

Review URL: https://codereview.chromium.org/2279603002 .