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
As a side-effect, the max_sad check is removed from the
C-implementation of VP8, for consistency with VP9, and to
ensure that the SAD tests common to VP8/VP9 pass.
That will make the VP8 C implementation of sad a little slower
but given that is rarely used in practice, the impact will be
minimal.
Change-Id: I7f43089fdea047fbf1862e40c21e4715c30f07ca
