Change test image format to VPX_IMG_FMT_I420

Code was previously using VPX_IMG_FMT_VPXI420, which was intended to be the
"vpx" non-YUV colorspace variant.

Change-Id: Icf8771eeefeb574055ed638a93450c3d0ed5b9f5

test/i420_video_source.h

@@ -83,7 +83,7 @@ class I420VideoSource : public VideoSource {
   void SetSize(unsigned int width, unsigned int height) {
     if (width != width_ || height != height_) {
       vpx_img_free(img_);
-      img_ = vpx_img_alloc(NULL, VPX_IMG_FMT_VPXI420, width, height, 1);
+      img_ = vpx_img_alloc(NULL, VPX_IMG_FMT_I420, width, height, 1);
       ASSERT_TRUE(img_ != NULL);
       width_ = width;
       height_ = height;

test/video_source.h

@@ -103,7 +103,7 @@ class DummyVideoSource : public VideoSource {
     if (width != width_ || height != height_) {
       vpx_img_free(img_);
       raw_sz_ = ((width + 31)&~31) * height * 3 / 2;
-      img_ = vpx_img_alloc(NULL, VPX_IMG_FMT_VPXI420, width, height, 32);
+      img_ = vpx_img_alloc(NULL, VPX_IMG_FMT_I420, width, height, 32);
       width_ = width;
       height_ = height;
     }