Add unit test for vp8_sixtap_predict functions

This unit test tests vp8_sixtap_predict function against preset
data and random generated data. The test against preset data
checks the correctness of the functions, and the test against
random data checks if the optimized six-tap predictor functions
generate matching result as the c functions. It tests the
following functions:
vp8_sixtap_predict16x16_c
vp8_sixtap_predict16x16_mmx
vp8_sixtap_predict16x16_sse2
vp8_sixtap_predict16x16_ssse3

vp8_sixtap_predict8x8_c
vp8_sixtap_predict8x8_mmx
vp8_sixtap_predict8x8_sse2
vp8_sixtap_predict8x8_ssse3

vp8_sixtap_predict8x4_c
vp8_sixtap_predict8x4_mmx
vp8_sixtap_predict8x4_sse2
vp8_sixtap_predict8x4_ssse3

vp8_sixtap_predict4x4_c
vp8_sixtap_predict4x4_mmx
vp8_sixtap_predict4x4_ssse3

Change-Id: I6de097898ebca34a4c8020aed1e8dde5cd3e493b