Don't decode AV1 images if AV1 decoder unavailable

A follow-up to PR https://github.com/AOMediaCodec/libavif/pull/3288. Fix
avifanimationtest test failures (avifDecoderNextImage() failed with
AVIF_RESULT_NO_CODEC_AVAILABLE) in the "build-static-av2 (OFF)" CI
workflow.

tests/gtest/avifanimationtest.cc

diff --git a/tests/gtest/avifanimationtest.cc b/tests/gtest/avifanimationtest.cc
index 8d37913..6490a47 100644
--- a/tests/gtest/avifanimationtest.cc
+++ b/tests/gtest/avifanimationtest.cc
@@ -86,6 +86,9 @@
   EXPECT_EQ(decoder->repetitionCount, AVIF_REPETITION_COUNT_INFINITE);
   EXPECT_EQ(decoder->image->exif.size, 1126);
   EXPECT_EQ(decoder->image->xmp.size, 3898);
+  if (!testutil::Av1DecoderAvailable()) {
+    GTEST_SKIP() << "AV1 Codec unavailable, skip the rest of the test.";
+  }
   for (int i = 0; i < 5; ++i) {
     EXPECT_EQ(avifDecoderNextImage(decoder.get()), AVIF_RESULT_OK);
     EXPECT_NE(decoder->image->alphaPlane, nullptr);
@@ -110,6 +113,9 @@
   EXPECT_EQ(decoder->repetitionCount, AVIF_REPETITION_COUNT_INFINITE);
   EXPECT_EQ(decoder->image->exif.size, 1126);
   EXPECT_EQ(decoder->image->xmp.size, 3898);
+  if (!testutil::Av1DecoderAvailable()) {
+    GTEST_SKIP() << "AV1 Codec unavailable, skip the rest of the test.";
+  }
   for (int i = 0; i < 5; ++i) {
     EXPECT_EQ(avifDecoderNextImage(decoder.get()), AVIF_RESULT_OK);
     EXPECT_EQ(decoder->image->alphaPlane, nullptr);
@@ -134,6 +140,9 @@
   EXPECT_EQ(decoder->repetitionCount, AVIF_REPETITION_COUNT_INFINITE);
   EXPECT_EQ(decoder->image->exif.size, 1126);
   EXPECT_EQ(decoder->image->xmp.size, 3898);
+  if (!testutil::Av1DecoderAvailable()) {
+    GTEST_SKIP() << "AV1 Codec unavailable, skip the rest of the test.";
+  }
   EXPECT_EQ(avifDecoderNextImage(decoder.get()), AVIF_RESULT_NO_CONTENT);
 }