Add VP9 frame-parallel unit test.

Make sure VP9 frame-parallel decode passes all the standard
test vectors. Only test running with 2,3,4 threads now.

Also refactor the video decode test driver to support passing
in decode flags which is used to enable frame-parallel decode.

Change-Id: I6a712464232c2e13681634951c7e176312522e1e
Merge "Add VP9 frame-parallel unit test." into frame_parallel