Fix a bug in adding frame parallel unit test.

There are two CreateDecoder functions and decode_test_driver is not
calling the right function now. This bug is discovered during really
enable the frame parallel flag inside libvpx. This bug does not affect
any existing unit test though.

Change-Id: Icd9633c4b66d50e422a09c4310ff791082878936
Increase the thread test range to cover 5, 6, 7, 8 threads.

Change-Id: Id25c294720551bb5153987d8758668befaa57929