vp9: Update to SVC datarate tests.

vp9: Update to SVC datarate tests.

With recent fixes to rate control for SVC the
buffer underrun in the tests does not happen,
so comment and TODO can be removed.

Also, in some of these SVC tests, replace the HD clip
with the corresponding VGA clip, which has > 400 frames.
For the (niklas) HD clip: it has only 60 frames but the
test was running up to 300 frames. Fixed it to 60 frames.

Keep some tests with the HD clip, needed for the 4 thread
and 5 level scaling test.

Change-Id: I0a2356a908e8b2271c7a422eb8b15c0d56eec968