To prevent the motion vector out of range bug, added a motion vector unit
test in VP9. 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.

The tests showed that this unit test could reveal the issue we saw before.

Change-Id: I0a880bd847dad8a13f7fd2012faf6868b02fa3b4