Add the multi-threaded first pass encoder unit test

Added the multi-threaded first pass encoder unit test in VP9. The test is
to check if the new multi-threaded first pass encoder(namely, new-mt = 1)
still generates matching stats. In the unit test, the new-mt mode will be
turned on once the multi-threaded first pass implementation is checked in.

Change-Id: Ic21bb1a55c454f024cfd2b397a4c148cfe638218

test/encode_test_driver.cc

@@ -226,6 +226,8 @@ void EncoderTest::RunLoop(VideoSource *video) {
 
           case VPX_CODEC_PSNR_PKT: PSNRPktHook(pkt); break;
 
+          case VPX_CODEC_STATS_PKT: StatsPktHook(pkt); break;
+
           default: break;
         }
       }