This unit test was ported from VP9. To prevent the motion vector out of
range bug, added a motion vector unit test. In the 4k video encoding,
always forced to use extreme motion vectors and also encouraged to use
INTER modes. In the decoding, checked if the motion vector was valid,
and also checked the encoder/decoder mismatch.

Change-Id: I1c72ea7c32a3cec9c67f1bbdc168e60507e57516