Use separate intermediate RGBA buffers in planar function tests

The existing tests reuse the intermediate buffers between the reference
and optimized implementations. In particular the existing tests appear
to pass even if the optimized implementation is completely empty, so
long as it does not modify the desintation buffers since these are
already filled with correct values from the reference code.

To avoid this, allocate separate buffers for optimized and reference
implementations to store intermediate data between function calls.

Additionally remove unused buffers from HalfMergeUVPlane_Opt tests.

Change-Id: I7e9ea21fc193e7be21cc24e2be0d7a122e068f6e
Reviewed-on: https://chromium-review.googlesource.com/c/libyuv/libyuv/+/6074941
Reviewed-by: Frank Barchard <[email protected]>

Author: georges-arm