Adds a test for the VP8E_SET_SCALEMODE control

Tests that the external interface to set the internal codec scaling
works as expected. Also updates the test to pull the height from
the decoded frame size rather than parsing the keyframe header,
in anticipation of allowing resolution changes on non-keyframes.

Change-Id: I3ed92117d8e5288fbbd1e7b618f2f233d0fe2c17