use_base_mv assumes 2x2 scaling, so fix is to shutoff
this feature unless spatial scale factors are 2.

Added svc unittest for 2 spatial layers with 5x5 scaling,
which generates the issue without this fix.

Also fix some settings in svc unittest:
let the speed setting vary (from 5 to 8), and enable static threshold.

BUG=webm:1344

Change-Id: Idfd0a6c633c21b49a0479601506302cfe974e30e