VpxEncoderThreadTest was taking a very long time for some runs and
timing out a lot.   This is an attempt to split the test into runs
that can be run nightly ( speeds 2 through 9) and runs that can
be run weekly ( speeds 0-1 ).

Change-Id: Iee6f61a561006d3a30381dd3b52b9a4dce07a70c