Add the motion vector unit test

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
diff --git a/av1/encoder/mcomp.h b/av1/encoder/mcomp.h
index 780b2d5..8465860 100644
--- a/av1/encoder/mcomp.h
+++ b/av1/encoder/mcomp.h
@@ -98,6 +98,8 @@
 extern fractional_mv_step_fp av1_find_best_sub_pixel_tree_pruned;
 extern fractional_mv_step_fp av1_find_best_sub_pixel_tree_pruned_more;
 extern fractional_mv_step_fp av1_find_best_sub_pixel_tree_pruned_evenmore;
+extern fractional_mv_step_fp av1_return_max_sub_pixel_mv;
+extern fractional_mv_step_fp av1_return_min_sub_pixel_mv;
 
 typedef int (*av1_full_search_fn_t)(const MACROBLOCK *x, const MV *ref_mv,
                                     int sad_per_bit, int distance,