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.h

@@ -218,6 +218,9 @@ class EncoderTest {
   // Hook to be called on every PSNR packet.
   virtual void PSNRPktHook(const vpx_codec_cx_pkt_t * /*pkt*/) {}
 
+  // Hook to be called on every first pass stats packet.
+  virtual void StatsPktHook(const vpx_codec_cx_pkt_t * /*pkt*/) {}
+
   // Hook to determine whether the encode loop should continue.
   virtual bool Continue() const {
     return !(::testing::Test::HasFatalFailure() || abort_);