Modify the encoder multi-thread unit test

Modify the encoder multi-thread unit test

Modified the encoder multi-thread test so that it included cpu-used=0 and
frame-parallel=0.

frame_parallel_decoding_mode is 1 by default, which disables probability
updating and gives lower encoding quality. Current VP9 multi-threading
encoder and decoder support probability updating. To test this part, we
should turn on it in the unit test, namely, setting frame-parallel to 0.

Change-Id: Ia1f86e01f0de628f50d819ae31509de3e1b6c755