The unit tests for VP9 multi-threaded encoder are added, which
carry out tests for all three modes(i.e. kTwoPassGood, kOnePassGood
and kRealTime), and speeds ranging from 1 to 8. A 1280x720 test
clip is used, which is encoded into multiple tiles. The number of
threads is num_of_tiles.

Change-Id: I04419eeca145ad841c9c527603668239a82e7fbd