added encode/decode matching validation to tests

This commit adds the ability of validating matched encoder and
decoder to unit tests.

Change-Id: Ie00d69a42477b6a69b324a6bd134939684f7300b
diff --git a/test/encode_test_driver.h b/test/encode_test_driver.h
index d9a79ec..3d74b74 100644
--- a/test/encode_test_driver.h
+++ b/test/encode_test_driver.h
@@ -52,7 +52,6 @@
   vpx_codec_iter_t  iter_;
 };
 
-
 // Implements an in-memory store for libvpx twopass statistics
 class TwopassStatsStore {
  public:
@@ -92,6 +91,9 @@
     return CxDataIterator(&encoder_);
   }
 
+  const vpx_image_t *GetPreviewFrame() {
+    return vpx_codec_get_preview_frame(&encoder_);
+  }
   // This is a thin wrapper around vpx_codec_encode(), so refer to
   // vpx_encoder.h for its semantics.
   void EncodeFrame(VideoSource *video, unsigned long flags);
@@ -128,7 +130,6 @@
   TwopassStatsStore   *stats_;
 };
 
-
 // Common test functionality for all Encoder tests.
 //
 // This class is a mixin which provides the main loop common to all