Remove Wextra warnings from vp9_sad.c

As a side-effect, the sad unit tests for VP8 and VP9
had to be separated.

Fixes a bug in original patch:
(https://gerrit.chromium.org/gerrit/#/c/70163/8)
that was reverted due to a nightly test failure.

Change-Id: Ia2a4e9e278fd3c89d6c3c82fcc6381320ec2a8a6