Use ImagePtr for ret val of testutil::CreateImage

Use the ImagePtr type explcitly instead of auto for the variables
storing the return value of testutil::CreateImage().

A follow-up to https://github.com/AOMediaCodec/libavif/pull/1069.

tests/gtest/avifprogressivetest.cc

diff --git a/tests/gtest/avifprogressivetest.cc b/tests/gtest/avifprogressivetest.cc
index 641a9d9..37f5ff9 100644
--- a/tests/gtest/avifprogressivetest.cc
+++ b/tests/gtest/avifprogressivetest.cc
@@ -157,7 +157,7 @@
     GTEST_SKIP() << "libyuv not available, skip test.";
   }
 
-  const auto image =
+  ImagePtr image =
       testutil::CreateImage(kImageSize, kImageSize, 8, AVIF_PIXEL_FORMAT_YUV444,
                             AVIF_PLANES_ALL, AVIF_RANGE_FULL);
   ASSERT_NE(image, nullptr);
@@ -192,9 +192,9 @@
   encoder_->width = kImageSize;
   encoder_->height = kImageSize;
 
-  const auto smallImage = testutil::CreateImage(
-      kImageSize / 2, kImageSize / 2, 8, AVIF_PIXEL_FORMAT_YUV444,
-      AVIF_PLANES_YUV, AVIF_RANGE_FULL);
+  ImagePtr smallImage = testutil::CreateImage(kImageSize / 2, kImageSize / 2, 8,
+                                              AVIF_PIXEL_FORMAT_YUV444,
+                                              AVIF_PLANES_YUV, AVIF_RANGE_FULL);
   ASSERT_NE(smallImage, nullptr);
   testutil::FillImageGradient(smallImage.get(), /*offset=*/17);
 
@@ -212,7 +212,7 @@
 }
 
 TEST_F(ProgressiveTest, ResolutionChangeExternalWithAlpha) {
-  const auto image =
+  ImagePtr image =
       testutil::CreateImage(kImageSize, kImageSize, 8, AVIF_PIXEL_FORMAT_YUV444,
                             AVIF_PLANES_ALL, AVIF_RANGE_FULL);
   ASSERT_NE(image, nullptr);
@@ -222,9 +222,9 @@
   encoder_->width = kImageSize;
   encoder_->height = kImageSize;
 
-  auto smallImage = testutil::CreateImage(kImageSize / 2, kImageSize / 2, 8,
-                                          AVIF_PIXEL_FORMAT_YUV444,
-                                          AVIF_PLANES_ALL, AVIF_RANGE_FULL);
+  ImagePtr smallImage = testutil::CreateImage(kImageSize / 2, kImageSize / 2, 8,
+                                              AVIF_PIXEL_FORMAT_YUV444,
+                                              AVIF_PLANES_ALL, AVIF_RANGE_FULL);
   ASSERT_NE(smallImage, nullptr);
   testutil::FillImageGradient(smallImage.get(), /*offset=*/17);
 
@@ -277,9 +277,9 @@
   encoder_->width = kImageSize;
   encoder_->height = kImageSize;
 
-  const auto smallImage = testutil::CreateImage(
-      kImageSize / 2, kImageSize / 2, 8, AVIF_PIXEL_FORMAT_YUV444,
-      AVIF_PLANES_YUV, AVIF_RANGE_FULL);
+  ImagePtr smallImage = testutil::CreateImage(kImageSize / 2, kImageSize / 2, 8,
+                                              AVIF_PIXEL_FORMAT_YUV444,
+                                              AVIF_PLANES_YUV, AVIF_RANGE_FULL);
   ASSERT_NE(smallImage, nullptr);
   testutil::FillImageGradient(smallImage.get(), /*offset=*/17);